Maybe he is at that akward stage where he is longer and taller than wide. When I use to have German Shepherds my male pup looked skinny because he was growing longer than wider. Kids are the same way. Your dog looks fine to me. My female lab who is 6 yr. old weighs 65lbs. She looks great. I would not try to pack on pounds during this time it might put undue strain on his growing bones. Before I adopted Koko she weighed 85lbs with previous owners. I really think that is what messed up her knees.
I disagree with your vet. My guess is that your Lab will weigh around 80-85 lbs./36-39 kg when reaching adult weight although, since there are so many different growth curves for Labs, that's a wild guess.
There's been more than a few recent studies done on Labrador Retrievers which found that Labs fed a normal amount of food as puppies and throughout their lives had significantly shorter life spans plus significantly more health problems than a group of their matched littermates fed only 75% of that amount.
So, if one has to err, it's far better to go to the lesser side.