I second the opinion to take yout kitty to another vet. I am wondering if tests have to be done?
I have a cat that had chronic diarrhea for years. The vet could never get it under control with medication. He just said he had a sensitive stomach and he recommended I feed him Royal Canin for Sensitive stomanchs from his clinic. So, for years his diarrhea would come and go - poor kitty!!! I felt SO bad for him....
I finally got fed up and started my cats on a raw diet. Within a couple of weeks of the new diet the diarrhea completely cleared up and has not returned in the 5 months I have been feeding him raw. I am now convinced that cats DO NOT handle Grains (rice, corn, wheat, etc.), fillers, perservitives, meat by products, etc. very well. And many cats are allergic to all these wasted ingredients. Just the feedback alone from fellow raw feeders with the same experience has me convinced
This may not be the case for your kitty but it may be worth a shot??? Even if you try half canned and half raw to see if there is a bit of an improvement. Also, now if I do feed him canned it is GRAIN FREE. I use Wellness canned. If you want maybe start with GRAIN FREE canned (I didn't see grain free mentioned above)