If he’s been checked by a vet and nothing is found (he’s just being picky) try the suggested above, egg (raw or cooked), some raw (thaw slowly in fridge) or cooked chicken prepared properly as a topper. You can also try raw goats milk to add some fat into his diet, or look for new born puppy dog formula at a pet store and mix that’s with his food. Also whatever brand he is eating try to see if it has a puppy formula and feed him that if he likes it, you can also top with canned puppy food as well for a calorie boost.
The main the would be to make sure he doenst have any ailments first. And hopefully getting him transitioned onto a better quality food.
