I would try switching things up and trying some new types of food. Not necessarily the brand, but if they make chicken/beef/lamb/etc try feeding a different type each week. People make the mistake of assuming that dogs will eat anything all the time. Although it's usually true, some dogs want variety or straight up don't like certain types of food.
You may also want to try feeding him out of something different than his usual bowl OR putting his food in a different place in the house.
You may also want to try feeding him out of something different than his usual bowl OR putting his food in a different place in the house.