I second what badger says about the slippery elm bark. I had a cat with severe IBD and none of the 5 meds that he was on made a dent in his diarrhea. It wasn't until I gave him some SEB that he finally had a normal poop. Some info for you on that:
http://www.littlebigcat.com/?action=...em=slipperyelm
It's really important that you get him to eat. What other foods have you tried? I would definitely stay away from any of the Hill's/Medi-cal products from the vet's office. Across the board, these have crappy ingredients not appropriate for cats. Here is some good info on nutrition, especially as it pertains to cats with messed up digestive systems:
http://www.catnutrition.org/index.php