Having been obessed over the wieght of my very health dane, I can relate totally. I would have him vet checked again, in case of worms or an underlying problem. Once he gets a clean bill of health from them I would look at his behavior. Is he happy, energetic..pro perly "puppy rotten"? Good coat and skin, shiney eyes? Puppies all go through the "uglies" when they grow too fast, get skinny and then rest, fatten up and wait for the next growth spurt. An egg, a little cottage cheese, yogurt, or a couple of teaspoons of canned food in each meal could increase his appetite nad give him a bit of a boost. As long as he vet checks fine and behavior indicated a healthy puppy I wouldn't worry too much.
Stella