Chocolate Lab Size

Question
I have a male, non-neutered, chocolate lab that is right around 6 months old. We are constantly queried about his age and size. People feel that he is too small. He weighs about 35 lbs and between 17-18 inches tall and is lean but not overly so. We are very active so he's very fit. His Dad is about 75lbs. and his Mom about 65 lbs. I'm just curious if it's possible that he is destined to be small or if it's still possible for a growth spurt to occur?

Answer
My guess is that he is going to be small.  Keeping them lean does slow growth, which is the best possible thing for a large breed puppy's joints.  Some dogs do continue to grow a fair amount after 6 months.  See http://jn.nutrition.org/cgi/content/full/134/8/2027S