They don't have anything like Imodium for cats that you could try him on? That is what I take when I get the flying sh******.....and it helps sometimes, sometimes it don't and required a stronger anti-diarrhea from the doctor. Looks like they could do something...

Have you tried cooking him stuff at home like chicken and rice, skimming off ALL the fat, put in a blender and puree it? It could be a ingriedent in the bought stuff that sets him off?