Your dog will bulk up naturally as he matures. As others have already mentioned, bulk should be muscle, which is developed with maturity and exercise, not fat. It's unhealthy to fatten up any dog as even just a few extra pounds is a lot of added stress to the body. I think it's even worse to add extra fat on an immature frame.
Judges should be looking for the whole package not just substance. If your dog is a quality specimen, there will be judges that will recognize and reward knowing full well that he is still a growing puppy.