Providing you are feeding the right kind of dogfood, that is food made for a large breed and for a junior dog, he'll be fine. I assume that he gets enough ( double check on his body weight; he'll eat a surprising amount at that age) and that he is fed at least twice a day At 18 months he is still growing and will fill out just be grateful you haven't got a Wolfhound ; there a male grows until 3 years though the bitches are grown by 2 1/2 . That's why I say that a GSD is likely growing still; males of larger breeds, not just giant ones, do grow on quite a bit even when they've reached full height