A German Shepherd will be fully matured by the times he is three or three and a half years old. He can now assert himself and if he finds he can subdue the family members by displaying aggressiveness, he will not stop unless trained to be otherwise. Large Dogs like German Shepherds, Dobermans, Rottweilers, etc. need very intense training to be disciplined, otherwise they can get dangerous. Please contact a good Dog trainer. Neutering can help to a certain extent, but I do not believe it will eliminate his aggressiveness completely.