Hi there,
There could be many reasons why your GSD seems to be more agressive as he matures. Has he been hip scored? Is he neutered? Has he had a recent health check?
If you rule out a medical reason and he has been neutered (it's never too late to see positive results although some form of trainer or behaviour specialist will be needed to break a habit) and you would like some ideas please supply a few more details, When did this start? Is he aggressive to people, dogs or both? Is he aggressive at home in any way? Do you have worries when he is off the leash? That sort of thing and I will try to help