Maybe he needs a food that has a higher fiber content in it. Have you tried giving him canned pumpkin at all? My two latest additions, Alani & Blaze have always had very sensitive tummies but they were always able to make it to the litter boxes. I now feed them Chicken Soup For The Cat Lovers Soul in the dry light formula because it has higher fiber than the adult formula. It's really helped them a lot. I do give them a little canned mixed in with their dry only about 2 times a week and they also get some chicken broth almost every day. I hope you'll be able to find a solution soon. Good luck.