Hi there, when you take him in the garden walk around the garden with him don't just leave him to it, also at this point teach him the command 'be quick' all my dogs 3 of which are GSD's pee and poo on command, walk around the garden with him saying 'be quick' at short intervals then when he has a pee say it quickly again and give praise, don't overdo the praise as GSD's as a rule don't appreciate it, they are not very demonstrative (big word this early in morning lol), you will find he learns this command very quickly, then each time you take him out all you have to do is say be quick and if it is raining and late at night you won't have to be walking around getting soaked for ages.
Lots of luck....................I want one!!