When I got Bosco, he had similar issues, and after many trips back/forth to vet he was diagnosed with giardia and treated with pancur. It did solve the problem, but it took about 2 weeks. Don't be surprised if the diarrhea returns (I hope it doesn't!), but giardia is difficult to get rid of and hard to detect. I hope you are walking him outside your yard, as giardia (if that is the culprit) can start breeding in your yard.
Bosco is now about 1.5 years, but I definitely know that he has a sensitive tummy. Took us a bit to find a good food for him, and I am very careful with treats. Keep us posted. I remember those initial weeks with Bosco very well, and the worry that went along with it.