I've had over the years three cats start with a bloody end to bowel movements and later be diagnosed with a form of IBD. Tylan powder was prescribed for two but I only had short lived good effect from its use. Did have some short lived good results with Metronidazole. Diet adjustment and long term use of the corticosteroids was the best to suppress symptoms of IBD. There is a lot that I could say about it but I just now read these few linked articles that are a good read on the subject. The first cat that I had that suffered so terribly with diarrhea was found to be infected with an uncommon cocci that was not detected on repeated in house fecal labs. I stress that you be sure the cat is clear of any problem parasites as a first step. Here is a good article to read here that has a lot of info for dogs but is good for what there is for cats:
http://www.dcavm.org/02oct.htmThere is a water soluble Tylan. My neighbors have used in their bee hives but its probably off label for cats. Your Vet could possibly get it. I had no luck mixing the Tylan powder into the cat food either. I had a Vet Pharmacist fill it into tiny gel caps which worked very nicely with a pill plunger. The Vet was hopeful that the Tylan taken along with prednisone would have good results but it didn't for my two cats. Another short article linked here is good:
http://www.vlsstore.com/Media/PublicationsArticle/PV_23_11_964.pdfYou say you are giving him 5mg prednisone 2/day for 3 days? The shortest run on that for my cats was always been at least 2 weeks and then 2 weeks tapering off. Just 3 days at full dose seems too short of a time to see any good effect.
As for good cat foods to try with a medication I lately like Tiki Cat Ahi Tuna of which there are several combinations which are very good. Best for my cats has been my own home made baked Chicken which I prepare very much like a baby food. Other wise I do use Gerbers plain meat varieties. I never got any cat to accept the Hills Z/D which I hear is very good for cats with IBD.
Sorry to read about this bad turn you are having with him. Ivan? If there isn't any vomiting along with a bad bowel I would be hopeful that he can improve soon. Has there been any mucus like substance along with the hematochezia?