I am not a GSD expert, but in general dog go through the ugly stage in their life when they are growing and can look thin and leggy, His height etc looks ok for a GSD. If he is active and healthy otherwise then he is fine. He should be also deformed every 3 months. The tablets that you get is by body weight so ask your vet. there are quite a few available.
Another way of seeing whether your dog's weight is ok or not is by feeling his ribs. If the ribs are not visible but can be felt by pressing a little then its ok, if you cannot feel the ribs the dog is over weight. Generally GSD's are not really fat dog and are mostly lean working dogs as far as I know and see.
Height of the dog is taken from the ground near the front paw to the shoulders.