Booboo is confusingly not well

I am taking this cat to the vet shortly (this am) and will update when I get back. I was wondering if anyone can put these pieces together, because he is confusing me to all grades of crazy!
My now 15 year old neutered & P/U'd male Persian has lost a surprising amount of weight, but we had thought it was from a rotted tooth. He had given up on his dry food and only wanted wet. We feed only feline C/D (Hill's Prescription Diet, per vet instructions because of crystals and blockage, hence the P/U) He has been doing fine on that, until his tooth, which I had extracted. He was on three rounds of antibiotics afterwards because the extraction site was still a little red and there is STILL a white scabby looking thing over the site (Extraction was done 2 months ago)
I am worried it is kidney failure, but he is not vomiting, he is peeing, he is drinking his water but not constantly, or "too much", pretty much the same there. He is not pooping very much but has not eaten very much. At times he will walk a few feet and lay down like he is exhausted, then he will go bounding up the steps like a kitten!
He scratched on his 2x4 (Their scratching post) today with all of the normal energy he usually has. Then he stands around looking as if he'd just gotten back from being put under at the vets, kind of disoriented. He also has a creamy/very light yellow discharge like snot from his eyes. I use a Q-tip at the inner corners and roll the q-tip (Never touching his eyeball) and it comes out just like SNOT. It's very strange. His ears are not hot, his nose is not hot, though he did have the sniffles and sneezed on and off for about 4 days (So did my younger female cat, though).
I am just so confused and will update when I get back from the vet. I've never had to put a pet to sleep and it is extremely terrifying for me.
Thank you for taking the time to read this and/or respond to any of it.

Had to leave Booboo at the vet for a few hours. Vet says he is dehydrated, constipated and possibly anemic from possible kidney impairment. He is getting an enema, sub-q fluids and a blood test to see what is going on. I feel like the world's worst furbaby mom ever for not taking him sooner. Vet seemed to be a bit upbeat and told me it was manageable depending on the results, and they would show me how to do sub-q fluids at home (so scared of that) and possibly change his food to a kidney diet food. I've read that a lot of cat owners horribly dislike C/D (say it is junk, I don't doubt them there) but he's done so well on it up til now.... goodness IDK I am a wreck already.
Thank you for the vibes Aj, they are so appreciated! Will update when I get all the info back.

Thank you thank you thank you for all of the replies and good vibes. The vet did tell me that because he was dehydrated the numbers of his CBC might be a off a bit. His BUN was greater than 130, and his Creatinine levels were extremely high too. His Blood sugar was up slightly but he said he figures that is because of his dehydration. He went poop, he pee'd when he got home (refuses to pee at the vet, ever) and we have a couple cans of Hill's K/D and he DUG into that ferociously when he got home. He started salivating very heavily and scared me, so I called back and they told me if he's still doing it by 5 he'd have to come back up there. But he seems to be calming down now.
I am also a little miffed that they did not clean him up after his enema? I had to put him in the tub and get as much off as gently and quickly as I could before he started flipping out. He is drinking water and eating his treat and calming down now.
I will be taking him back each day around 9:30AM to get his Sub-q's and then once I'm comfortable with doing it myself I will do it.
The vet told me if his numbers don't improve after the next almost week of sub-q's, water and K/D wet food that it means there is not much we can do and we will "have to make a choice"
Thank you again for all of your replies and vibes.

Thank you again for your replies. I'm in the US, btw. Vet said they will do another blood draw at the end of this week (or beginning of next) to see how the fluids may have changed the numbers. Yes, I read that CRF site and still am looking at it, so much info and God Bless the person who put all of that together! I agree, treat the cat, not the numbers. This morning he is doing a bit better, he is purring, he stretched, and he is able to jump into his favorite rocking chair. His nose was pink when I got home but after about an hour it has gone pale again. The vet said this was a sign of anemia, but I believe during his whole life when he is still for long periods of time or it wasn't VERY warm in the house, his nose would go pale like that. I never thought anything about it. I did notice his gums when the vet showed me and they WERE pale. I will talk to the doc about doing something for the anemia and everything else. Also a sanitary shave (wish they would've done that before they gave him the enema) and to try and wash his tooshie so he doesn't get a UTI.
I will update if I get any new news or information. Blessings to all of you for being such kind hearted people.

I am definitely going to speak to the Vet himself about leaving him in his own poop. They know I am one of those cat momma's that want him home ASAP and I can't stand leaving him there by himself. I think they've become annoyed with me being how I am. I've overheard several nurses there gossiping about other owners. The Vet himself I like, and the staff gossips about him too, but I would prefer another vet in another county, but like I said, with Booboo going downhill like he was, I didn't think he could cope with a 30 minute car ride there and hours of wait time (That vet is -extremely- popular). I will also consult with the Vet about thyroid and blood pressure, along with the anemia. You all help to save lives I believe, and I thank you soooo much.

You are right I should not concern myself. I fear that any dislike of me will spill over to my cat, though. I always try to be as nice as possible. That being said, I watched the injection today, and... I cannot do it. I just cannot possibly do that to him. I thought it was a needle of fluid, not a 1000ml bag hung up and an IV drip! He got 200 ml today, and I had to hold him during it. He was NOT happy and I've never seen him get that furious before, it was scary. They used an "18" needle, whatever that is, as one of the Vet docs there said "at LEAST use an 18" (I do not prefer this doc, same doc that somehow misdiagnosed a fungus in his ears for "dog mites") Is that needle too large? I will do some research. I cannot do this to him. I was good to go right up until she showed me how to tent his skin and stick the needle in, then told me if it stops flowing I would have to move the needle around a bit and pull on his skin a bit, and... UGH I can't do it! Too much risk of him trying to injure me severely or cause me to inadvertently injure him, I will just have to pay them. I am also going to purchase some Feliway (45 dollars there, for a small spray bottle?) to calm him. In the future I may just have to go to 1 of the 2 other vets available that are further but are, in my opinion, more careful and caring. I will try to watch each time to get more comfortable. We will see >.<
As for the stool issue, the doc that always works on him was off today so I asked politely that he call me when he has a minute TOMORROW and that I needed to speak with him about a few things.
Good news though, Booboo's eyes are much wider, clearer and livelier, he did not drool or seem to get -as- stressed out (His butt was cleaned and shaved so that is part of his aggression at the vet) so maybe having to go every day for a little while will get him more used to it? I hope so. I really have to get some sleep
Again, thank you so much, I cannot express in words how much help you have all been and you are all priceless. Blessings to you! Will update if anything changes...

He has received 200ML of the fluids twice. This was the second day. I have to take him up there each day for the rest of the week for 1 treatment a day. I know I shouldn't jump to saying I can't do it, it is just so terrifying. What if he jerks and I puncture through his ribcage into his lung, or something horrible like that? I will try to keep my mind open and do it. It also took about 15-20 minutes for the fluid to get in there, and that was after setting everything up. It seemed to take forever today, but hopefully that will be a little better once I become more used to it.
I was upbeat when the nurse told me that he could -possibly- get to the point where he only needs it twice a week. They said they would lessen the ML given to under 200 once they think he was getting hydrated enough. But after talking to my friend who just lost a cat to cancer, telling me I need to think "about what is best for the cat," I am just depressed all over again. But I will try to stay positive! He is doing well for now and cleaning himself, and hopefully his tooshie will become not so tender soon. I know I can't take him up to the vet for the rest of his life, even 2 days a week, I guess I was hoping he would become used to it but it may cause more harm than good with all the stress.. I was told after this week's hydration we will do another CBC and see where the numbers are at, and talk about treatment for BP and everything else.
If it wasn't 200 ML I would be more open to it, 200 took so long it seemed!
Oh goodness. We will see! Thank you all for being so positive and helping me through this difficult time.
I WILL talk to the vet about Miralax! He went poop last night and it looked fairly normal except it was in balls instead of one long portion.
The prices they gave me for take home supplies we 50 cents a needle, 10 needles is 5 dollars, 1000ML of fluid was $22.00 and the extension set was 7 dollars which they said I can reuse. I would not be reusing needles.
Thanks again guys.