Thanks again everyone so much for the advice
Neville is actually doing much better today. I've put him completely on his wet food and he is no longer having complete diarrhea, just a little soft. He hasn't missed his litter box at all during this round of stomach upset (knock on wood), which is amazing and making it much easier. I've also been making sure he drinks plenty and his behavior is still completely normal. I'm planning on getting another can of pumpkin to make the freezer treats so that I have something a little extra to give him that's easy on his system any time he has a little episode.
Hi Reg - Sorry I missed that question before. He wasn't given any antibiotics after surgery. But they mentioned the anesthesia alone could make his stomach upset, so I'm thinking that might have played a role.
I'm not sure what exactly caused him to become so sensitive to diet, but now that I know that just wet food seems easier on him, that's a great place to start.
I'm thinking that anytime he gets anything slightly different in his diet (even dry food), his system can't handle it anymore. I'm very good about keeping him away from our food and from our other cat's food, but the other day when I got out of the shower, I caught him chewing on a piece of hay from our guinea pig's cage (which I took away immediately). I'm thinking that was just enough to cause this round of diarrhea, so I moved where the guinea pigs food is so that he can't try to pull it through the cage bars. I vacuum daily and always try to keep a close watch on him to keep this kind of stuff away from him, but he sure is sneaky!