It sounds like your cat likely has inflammatory bowel disease (IBD). As such, you should consider trying him on a hypoallergenic diet. A very good prescription feline diet for this purpose is IVD, rabbit and grees peas diet. Your vet should be able to order this food for you. While hypoallaegenic diet sometimes resolves IBD or makes it considerably better, sometimes drug therapy, at least periodically, is necessary in severe cases. This involves concurrent maintenance onf Flagyl, and seomtimes even the addition of a steroid like injectable depo-medrol or oral prednisolone. Please follow this link an article I wrote on IBD for more details:
http://www.maybeckvet.com/inflammatoryboweldisease.html
Good luck to you. Thank you for using JustAnswer. Please let me know if I may be of any further assistance.
Best of luck to you, and thank you for choosing JustAnswer! Please do not hesitate to let me know if I may be of any further assistance.
Sincerely,
Roger L. Welton, DVM
President, Maybeck Animal Hospital
Author, "Canine And Feline 101"