Hi Livster:
Welcome to the forum. Hopefully we can be of some help.
I was wondering if the wee guy has had his vaccinations and when were they given? And when was his neutering? And was he given any antibiotics either by injection or mouth - you didn't answer that question posed by Hazelrunpack.
I'm enclosing a web site giving you some idea on prescription diets by Dr. Pierson, who is a vet in the states.
I also agree that you should seek a second opinion. Possibly try to find a vet who is integrative. They are knowledgeable on both allopathic and alternative medicines and have a good working knowledge on nutrition - more so than the conventional vet.
Also try to make sure they specialize in felines.
There is a lot of good information on this web site.
http://www.catinfo.org/#Prescription/Therapeutic_Diets_
Something you might think about trying is Slippery Elm Bark. I've used it myself and have had good success.
http://www.littlebigcat.com/health/slippery-elm/
It would also be good to go into Article Library on Little Big Cat. There are a lot of really good pieces there that might assist you now and in the future.
Please keep us posted.