Hi lynx556-
Have you brought in a stool sample into your vet clinic to check for parasites, including giardia and coccidia? Worms and parasites are very common in young puppies.
Has the vet recommended adding a little fiber to the diet? I have a kitty who has suffered with chronic diarrhea for months. I recently started adding a little psyllium husk powder and a probiotic. It has helped immensely. Make sure to get direction from your vet on how much to add. Adding too much could be dangerous.
I would definitely stick to a large breed puppy food. You want to make sure his joints grow and form correctly. It contains the appropriate amount of calcium and phosphorus for a growing large breed.
