Have you heard of Nutrisource? It's a really good kibble. I've seen dogs with some pretty touchy stomachs and/or bad allergies do really really well on it. No fillers, all in-country made, etc....I'd recommend it highly. I am NOT trying to bash vets, but a lot of vets know very little about nutrition/diet. Allergies and diets can be exhausting and difficult to deal with. The only reason I recommend Nutrisource is because I've seen it work on some pretty messed up dogs (digestive wise). I would go on a high quality kibble, and cut out allll snacks (bully sticks, bones, etc) until you get solid stool. Then introduce things one thing at a time. Although, your vet (who knows history and your dog) might be better than internet folks. Good luck!
"An excess of size is an unserviceable feature for breeding, his powers of endurance, his speed and the smartness of his movements suffer in all circumstances. Giants are never nimble."-Creator of GSD