Consensus at the moment is that he has another infection. The skin on his belly is extremely thickened, and they tried to aspirate some stuff from under there to see what was going on, which pissed him off. One of the doctors said that it reminded him of the skin version of T-cell lymphoma, which he hasn't seen in cats, but he has seen in dogs, so I think they're going to look for that. Right now, they're going to treat him for another bacterial infection.
They also said that he was kind of dehydrated, so they gave him some fluids, which really pissed him off. I wanted to have them show me how to clip his claws, but we all figured he'd been through enough for one day. Next time, maybe, we'll do the claws.
When we got home, it did seem that he'd perked up a bit and he was eating like someone was going to take it away from him. I'm going to go out in a few minutes and do some grocery shopping, and get some gooshy food to put his pill in. But right now, he's looking much more lively, and giving himself a good bath, which I don't think he's done in a little while. And he hasn't chewed at his belly since we've been home, so I haven't put an e-collar on him yet. Tomorrow I'm going to swing by Petsmart and get him a bed and see if they have a proper e-collar there.
Meantime, I'm going to try to keep track of his behavior here so I can tell the vet next week when I talk to her again.