Food allergies usually rear their ugly heads
in pups under 1 year. You might try
a food elimination diet. try
a limited protein like natures recipe vegetarian or venison. This is the hard part no treat or anything but the food for at least 10 weeks to see if really helps. You can give carrots, canned pumpkin, marshmallow or banana for treats instead during this time. If that helps then easy fix and he'll be healthier enjoying those treats instead.