GI issue? Help!

Hello! I am new to the site and looking for some insights, experiences, and opinions!

My cat, Meeko, has been having troubles for a couple of weeks now. Please forgive me if this is lengthy, I want to make sure I give all of the information I can. 
It all started just after we got a new kitten, who Meeko has been getting along quite well with. It might also be important to note that he is the groomer of the bunch. He is always cleaning himself, our cat Mufasa, and the new kitten.
We've been to the vet a number of times, including one trip to the ER Vet.
His initial symptoms included gagging like he had to vomit and just being generally mopey. We had bloodwork done, and the only thing it showed was bands, and he received an antibiotic shot. Everything else was normal. She also did an x-ray to rule out asthma. 
Since then, he has been doing much better. He hasn't been gagging, but does kind of hard swallows when he's purring. He's been playing a lot, eating, and drinking just fine. 
I had increased his wet food when he was sick, as I was afraid of his fluid intake. We had run out of wet on Tuesday, and gave him his normal dry food on Tuesday and Wednesday. He vomited three times on weds, hours after eating. The kibble was basically whole, even though it was hours after he had eaten. There was also a small hairball in his vomit. We went back to the vet on Thursday, and I gave her a stool sample, still waiting on results. We were prescribed probiotics, metronidazole, and a dewormer. 
Since he vomited his dry food, I have been feeding him solely wet food until we get the stool sample results, because we didn't rule out a food allergy. 
He just gagged a couple of times, and vomited a little mucous. I fed him about two hours ago, but there was no food in his vomit. This is the first time he's done this. I'm not sure if it's his belly reacting from cutting out his dry food? I'm just afraid he has an allergy since the kibbles were undigested before. 
My vet still isn't sure what is going on. She definitely thinks it's a belly/GI issue.
Again, sorry this is so long! I just want to provide every bit of information. Please let me know if you need any additional information! Thank you all so much!

So he was fine up until a couple of weeks ago, and you'd been feeding him the same food all along?  Or did you change dry or wet foods somewhere along the way in the before he started showing these signs? 

What is "bands"?  You said his bloodwork showed he had that and was prescribed an antibiotic.  Sorry, I have no idea what that is, other than a musical group

Is he hesitant to eat now, or does he still have a pretty good appetite now that he's being served only wet food?  How has he done since you last posted?  Any vomiting at all?

Thank you so much for responding!

Yep, he was totally fine, and always on the same food. 

I'm not even 100% sure what bands are, she said they usually appear when there's a GI infection. 

He ate slightly less than usual this morning, though he still has a pretty good appetite. He also went potty twice today. He was quite gassy while going, and it was pretty loose, though not runny. Some snuck out right after he was done in the litterbox, and when I cleaned it up I noticed two small rice looking pieces (Not sure if they're worms or coincidence). I tried to investigate the second stool but didn't see any more. (Cat mom life!) I gave him profender dewormer last night around 9pm. 

He's sleeping right now, but they sleep most of the day anyway. He's also a pretty lazy boy, so Im not sure if he isn't feeling well or just being his normal bum self. He has also been grooming himself/the kitten  like normal. 

So he WAS vomiting, now that seems to be over and it's turned to loose stools?  That metronidazole is typically given for diarrhea issues   That, coupled with the rice like things you found in the box this morning makes me think he does (or did) have parasites, but still wondering about that gagging and undigested dry food issue.  Since he's the community Groomer, I wonder if he's got a large hairball stuck somewhere along the lines?  I think that might cause dry food not to be able to get past it to digest. 

Now that your thread has been bumped back up, maybe some of those more knowledgeable about this particular topic will see it and pipe in.  In the meantime,you're still waiting for the stool sample to come back, right? 

Kind of. He was throwing up undigested dry food the other day, was fine for a day or two, then last night started throwing up clear saliva. That happened three times in just about 24 hours. 

I've also been giving him the hairball medicine twice a week since Saturday, 6/27. He did throw up a very small hairball with the undigested kibble the other day.

I just fed them and he didn't eat much at all. Twice when he went to eat he gagged. But he eventually ate about half a plateful.

Yep, still waiting for the results. They're closed today so I'll be calling first thing in the morning.