Try switching him to a different diet. Some dogs can be pretty picky about their food. Some brands which I've heard to appeal to dogs (and are healthy for them too) are Innova EVO or Blue Buffalo. If dry food doesn't appeal to your dog, maybe you should try a BARF (all raw diet) or one consisting of cooked rice, chicken and vegetables.
You should buy food where the first ingredient is meat. They love the taste, if it still doesn't eat, try canned food. Mix the canned food such as Cesar with the dog food. That should satisfy your dog.