At this age, no reason to suspect any thing other than over angled and maybe loose ligaments.
If the dog is in no pain, I would not even do x-rays until ready to certify. No reason, from what you describe for the drugs either. Some vets get confused over the GSD rear, especially when over done.
Can you post a picture of his hocks? From the stand picture it does not look like he is really cow hocked. Some youngsters, especially large/tall ones, can go through periods of being cow hocked which many times corrects itself with maturity. Do keep him lean, on a lower protien diet, and NOT too many walks/runs on concrete (this can stress young joints, especially in a very large young dog). If he has no pain, he should NOT be on any pain medication/anti-inflamatory, which are in your case VERY unnecessary and can damadge his internal organs as well as potentially have horrid side effects. He should be on a glucosamine joint supplement of some sort if he isn't already. A lot of vets are not familiar with the structure of a German Shepherd, far less the structure of a developing German Shepherd. You should absolutley NEVER use that vet again, I would NEVER trust or use a vet who would prescribe pain/anti inflamatory meds to a dog NOT in pain!!! That to me is not a vet with the best interest of your dog at heart, but with the best interest of his/her bank account! Do his x-rays at 1 and get him hip scored, until then (unless he is showing signs of distress) enjoy your boy and try not to worry about it. If his cow hocks do worry you, bring it up to the breeder or discuss it with other GSD people or a vet that is knowledgeable in GSDs. BTW, your boy is just beautiful!!!! Stunning pigment, very good head and expression! Best of luck and keep us updated!