oh. poor kitty.
some suggestions (from my herbal cat care book, very informative) . wild yam is excellent anti inflammatory. slippery elm is also very beneficial. Dr Kidd, holistic vet says 1/2 tspoon per 10 pound body weight mixed with water and given as a liquid 4-5 times a day. although he says not to do it for more than 3 weeks at a time and then break for a week.
Also it may not be a bad idea to give him a teaspoon of yogurt a day. it says in the book that an excess of yeast can cause food allergy and what they call "leaky gut" yuck i know but it sounds a lot like what your cat is experiencing. probiotics could be exactly what he needs (plain unsweetend yogurt with probiotics or probiotics that you can get from a health food store. they should be able to give you the correct dose) . can you maybe suggest this to your vet? they may have some ideas.
hope that helps a bit.