I totally agree with crazy4cats. But, for a very sensitive stomach, I would like to add onto zcRiley suggestion.
Where I work, Zignature is one of the main options we would recommend when dogs have:
(a) unknown allergens but consistent allergic reactions to other brands (even the best such as Orijen Single-Protein diet), and
(b) stomach problems that aren’t clearing up with other single-protein/limited ingredient diets (possibly unrelated to allergies, just sensitive).
They specialize in limited ingredient diets that try to eliminate as many possible allergens and sensitives from their product while still being as nutritious as possible (note: probably because of this it is not a meat-based kibble like Orijen, unfortunately). I always recommend starting with Turkey or Duck (lean meats are easiest to digest) because it is the most economical and easy to find treats for if your dog has great success with it. I would recommend waiting on anything with Venison or Kangaroo as these are not only more expensive but useful if your dog has problems with everything else (i.e. last resort). You can also try pairing any new food with unpasteurized goats milk for the added digestive track probiotics (especially since its a puppy).
Note that any changes in diet will take a few weeks before noticeable effects may be seen with more time needed the bigger the dog (although your dog will probably see improvement after a week or so if its helping). Once you find what the problem is, you can transition them to a product that closely matches your dogs needs or stay on it.
Hope this helps! Wish you and your puppy much luck!
