Hi there...sounds like you have a bundle of energy on your hands!! First, let me congratulate you on raising a kid friendly dog who is NOT overweight. We rarely see the underweight dogs, so it is somewhat of a novelty!!
Next...what food are you feeding him and how often? Does he eat well?
21kg (or about 46 lbs here in the US) is a little small, in my opinion, for a year and a half old Shepherd. Are his ribs prominent (I know it will be hard to see with his coat)? Can you see the points of his hips? Of course, it is possible that he just has a small body frame too. Do you have any pictures of him you can post?
Also, you might ask this question over in the Ask A Vet Expert forum.
Good luck with him!!
He sounds like a lovely dog. Has he been neutered? The reason I ask is some dogs automatically burn off far more calories when they haven't been neutered. It is something to do with the hormone Testosterone.
I knew a dog who couldn't gain any weight. He was too skinny though well fed. When his owners had him neutered he filled out a bit more.
You can always change his diet a bit,by giving him a high energy food with extra protein,that may work