Sorry to hear about the troubled tummy...
Can't really help with the medical stuff, but when we got Sparky last year I went with GO! grain-free. It was too high on protein for him, gave him terrible gas and he never had a solid poop, it was always watery. Then we went with Orijen and the problems virtually disappeared. Almost no gas (rarely) and his poops are really nice (
When he had a real bad episode with diarrhea (before Orijen) the vet convinced me to buy the food from them (Iams Intestinal Low Residue Puppy) and just give it to him while he has tummy problem, going back to normal food after its gone. I was upset but went with it because it seemed to help. But thats how we figured it out, he was doing fine on Iams
, and then back to good quality food he was getting worse.
I would try another quality food, recommend Orijen, but do your own research, you might find other options too.
Good Luck and don't be afraid to try the vet's food. Sometimes it actually works.
Also, just wanted to add, with Tarzan RIP, we had to put him on Senior food relatively early, because he wasn't doing good on adult anymore(diarrhea or constipation), and senior was easier to digest and had glucosamine for his joints and all that...