I saw a second vet and he also suspects FIP/FIV/FeLV. He didn't mention the ELISA test... just testing individually. Also I think the test for FIP is really expensive, and it isn't 100% accurate either. He mentioned doing 2 tests... serum electro-something. He said it would see specifically which immunoglobulins are elevated. Also he wanted to do a blood smear. He's going to wait for me to call back if I want to do either of those. I think for now I'll just wait to see if the new diet and removing the clay kitty litter helps.
He thinks it's either that or possibly he was poisoned (maybe by the cat litter?). I will just have to wait 3 to 4 weeks to see if he gets better, and if he doesn't, it's probably one of the viruses.
He also mentioned testing for Lyme disease, but he didn't make it out to be a big concern. I read online that Lyme disease in cats can cause sudden lameless. My kitty was born in the springtime and whoever had him before had him outside sometimes. Anyone know anything about that?