What are you feeding him? If he's growing that fast, I''d be worried about Pano (Panosteitis, don't worry too much its not serious).....If feeding high-protein puppy food (most are), get him off the puppy food sooner (4 months or so)....Too much protein can contribute to pano....The aforementioned GSD had Pano when he was about 8 months-1 year....I didn't know about the too-much-protein, back then.
Cute little bugger.