How do you (or can you) predict growth on a German shepherd?

I've had a lot of German Shepherds and my one was the same as your boy - he weighed about 115/120. Your boy is at least going to be a 100 pounds, most likely. My Shepherd's parents were huge too- what about yours? You should be okay - if the vet says he is perfect weight he is (the vet is basing it on his frame and how much weight he should be carrying).