It sure sounds like he is a good eater! Just like Rexy!
Even though Rexy loves to eat - I do keep dry food out all the time and he doesn't overeat. I feed him and Rowdy a packet of nutro natural choice wet food every morning (I split a packet between them) along with some cottage cheese and then I leave dry food out all the time.
Maybe you could increase the dry food or free feed for a few days to see if he gains some weight! Because he is a puppy, he may be just burning it all up! He may be like some people, he just has a higher metabolism. Maybe you could give him some meat (pure protein) - like boiled chicken or I give Rex and Rowdy the toddler (human) chicken weenies (looks like vienna sausages - it's in the baby food section). They both love them and they are high in protein - which puppies need!