Does he do well on a home cooked diet? Have your tried a bland diet like a doggie stew of broth, chicken, white rice and maybe soft cooked vegetables?
Our dog was at the vet every three weeks for diarrhea. I did some home cooked food for almost a week and realized she did well on it. The issue was I was concerned about her getting all the supplements and vitamins. I found this website and read about raw feeding. I was a bit afraid to try it, but a few others had mentioned the Honest Kitchen brand, which is dehydrated raw and that basically cured our problem.
It seemed our dog could not handle kibble - it may be the texture was too upsetting.