Does he take treats (high value)? Maybe I have bad luck with puppies, I have had several go off their food here and there (even my adults do it). If everything else was still normal (no change in behavior, activity level, normal breathing/pulse, normal drinking, etc.), I learned not to panic, and I have dogs who have contests to see who can finish first. NOT saying I don't recommend vet advice, because I would consult with them, but IME, it's not all that unusual.