My dog used to be a really picky eater too, which is very odd for a Lab. I actually had to make a game out of feeding time by putting his food into treat balls and letting him chase the balls around for the reward. If he did go a few days without eating I would give him rice and scrambled eggs so he did get something in his tummy. If you’ve tried pouring tuna juice on your dog’s food and he likes it, then consider trying a food that contains fish. Or try a vegetarian food if your dog he likes veggies. There are a few brands out there that offer vegetarian formulas.