My old fellow was about 36 kg at that age and he filled out and matured at 50 kg.
Some lines of GSD grow faster than others. But really a slow growth is better for the formation of sound bones. Keep him nice and lean if possible so he does not carry too much weight around.
My current puppy is now 13 months old and around 40 kg and he is in a fit condition. I expect him at full maturity to weigh about 42 kg as he is not as big boned as my old fellow.