I would go to the vet and rule out hip/elbow dysplasia. It's VERY common in GSDs --- it happens all the time. Out of all the breeds, GSDs get it the most. It can be quite serious, so I'd get that checked out. Otherwise, give him glucosamine supplements (and/or chondroitin or shark cartilage) to help strengthen the ligaments and prevent dysplasia. I'm not sure of the exact amounts, but I do know that the stuff you buy for dogs is usally pretty useless. The stuff made for people is better.