Sick Cat even after several vet visits!! Help!

Hi. I have two grey male, neutered cats that we adopted from our veterinarian almost 2 years ago. They have always been healthy and playful kitties. I guess it all started slowly about a year ago I noticed that one of them, Wendell is his name, started acting like he didn't feel well, sleeping more often and just not acting like himself. Then I noticed the next day that the white eyelids on both eyes had come up over the eyes. Anyway, I took him to the vet that we have always used and he said he just thinks that he has problems with hairballs. (He's not a long-hair cat but more medium-length) so he gave me some hairball treatment stuff and I started giving it to both to them. He seemed better but it was then that he started this cycle where he was seemingly normal most of the time and I would say maybe one day out of a month, he would act like he felt terrible and would hide and not eat or drink and then the next day he was back perky and normal. I do not have tons of money and since I had just spent over $200 already I said well I since he's better the next day I will just feel it out and see...well that went on for months where just every now and then he'd have a bad day like that. So then I noticed a few months ago that the timespan between the bad days was getting shorter and he started to have weak, sick days once a week so I took him back to our vet, who once again stated he thought it was due to "hairball" symptoms and preceded to tell me how many cats he has seen have serious complications from a trapped hairball. It was around this time I also noticed he had lost weight and brought that up but the doc said he hadn't lost too much. All in all the vet seemed to act like nothing was really that wrong with him. But just to be sure, he gave him a few vitamin shots and sent me home with an antibiotic and parasite medicine to give him for a week. So I did this and he seemed SOME better but definitely not back to normal. So about 3 weeks ago he got to where he wouldn't eat or drink and was very weak so once again, I took him back to the vet and told him something is DEFINITELY wrong with him and demanded he do tests. He did bloodwork, tested him for feline leukemia / aids, and did a fecal test and all were normal. According the the vet, he doesn't have diabetes, his liver / kidneys and all were normal, and he doesn't have an infection. Which I was blown away because I expected him to find SOMETHING. So, once AGAIN he gave me some laxatives and said to give him some wet food that he likes to plump him up for a while and change his dry food to some prescription food and he should start to feel better. So I've done this and if anything he's worse. I'm now out over $600 and still my cat is sick. So to update this past week, he sleeps ALOT, doesn't play at all and has gotten very skinny. He still drinks water and when I try to feed him, he will eat but small amounts. I'm researching taking him to a different vet but in the meantime, I wanted to see if anyone had any ideas???

The top pic below is of Wendell, taken about 5 or 6 months ago..even then you can see the white part of his eye slightly. The bottom pic is of his brother, Clark and I'm happy that he's always been healthy and playful!

They are both right at 2 years old. When we adopted them almost 2 years ago they were only a few months old, if that.

Right now, I'm feeding him wet food because he hasn't touched the dry food that they both usually eat. Or the new dry food that the vet told me to try with them..I believe it's RoyalCanin. Their other dry food they ate before was Purino One Smartblend and they have always liked that. Our other cat, Clark will eat whatever you give him and he even refuses to eat the new prescription RoyalCanin so it must not taste very good to them.
Anyway, he always gets up to eat the wet food and sometimes he will eat actual bites and eat most of it and then sometimes he just licks it. I've found with him, he's always been a picky eater even back when he was healthy so I had already honed in that he only likes tuna flavors of wet food, no other meats. So that's what I've been feeding him is different flavors of Fancy Feast tuna wet food.
However, since a few days ago he still drinks water and will get up to eat but now only just licks it some and then gets back on the bed, or whereever and goes back to sleep.
So I want to try giving him some boiled chicken and maybe rice tonight to see if that will rev up his appetite / make him feel a little better. And if no improvement, I'm taking him somewhere else tomorrow morning.

So I boiled some chicken and cut it into very small pieces and spooned some of the broth from it onto it and was very confident he would at least eat this since it smells good..but to no avail. He wouldn't even touch it. He licked one piece. Not good..I would've for sure thought he would eat this.

Kas - Yes his hair is looking more greasy why do you ask? I just assumed it was because since he hasn't felt very good, he hasn't been grooming himself..and his hair is a mess. I've brushed it but he doesn't look good.

Another thing I forgot to mention. Not only is his third (white) eyelid present but sometimes (today) his pupils are different sizes.

He did drink alot of water when I got home today but I had to physically pick him up off the bed and bring him to it and then he drank it like he was very thirsty.

Was doing my own research on cat symptoms and I've really started worrying myself because even though he did test negative for Feline Leukemia Virus, he has alot of symptoms of types of cancer, just without a knot present that I can see.

Will be taking him to a different vet early in the morning. Does anyone know if veterinarians will give you a copy of the bloodwork they ran / tests? I really don't want to shell out that money for that again at the new vet and I'm sure they will want to see bloodwork.

His gums look normal as far as I can tell, he wouldn't let me look long. They aren't pale. But, I did just see where he is laying on the bed, he has a small area of drool and it smells very sour almost like what throw up smells like in humans but this is just clear.

Going to the vet in a few - Will let everyone know what they say. But, KAS - I looked up FIP and he has the exact symptoms for that...so that's really not good. He even had some neurological / disorientation signs a few months back.

Well, the vet said FIP can be difficult to diagnose 100% but he did think he has it and he did bloodwork, which showed he was going into kidney failure and liver failure because of the disease. So, he was so sick that he told us he wouldn't bounce back from this. We ended up decided to put him to sleep because I couldn't bear to watch him decline even more. So thank you for all your help and advice. Even though we had him only 2 years, he will definitely be missed!