He's a handsome fella, Brat!
We've never had a cat with IBD, but our dog, Evan, had it. We were told that IBD was like an allergy of sorts--novel proteins were initially okay, but then the dog would develop an intolerance for that protein, which resulted in intestinal inflammation and diarrhea. One of the first things they did with Evan was put him on prednisolone. It's a harsh drug and has a lot of side effects, but it worked--it reduced the inflammation and we got our Evan back. After a few weeks, we were able to wean Evan off the pred and control his IBD with diet alone. You might want to ask the vet if pred is ever used in cats with IBD?