So having experienced all the million stomach disorders in dogs, I would opt for a single novel protein diet, Tylan instead of
Metronidazole (you can
buy it in bulk at KVvet supply and put it in empty capsules), test for Giardia (use the blood test), and ask for Pred.
If the dog is allergic to certain proteins, the very last thing I would give is Chicken, beef, and maybe fish.