Even though he is a big puppy, treat him as a puppy. Try to get him used to praise so he knows what it means when it comes from you. When you are in the house and he comes to you (even on his own) say in a quiet voice, "What a good boy! Good boy!" You can use any verbage that you are used to, but use that tone. Try to find any reason what so ever to praise him. If you glance over at him and he looks at you, praise him. You don't have to be loud and throw a party if that causes him to shy off.
This will be a good time for you to get him used to the crate since he's coming home from the vet and needs to be quiet for a few days. Remember, EVERY time you take him out of the crate, take him directly outside. Keep him on a leash. Walk slowly around (to discourage him from sitting down and waiting on you) and you can say to him in a light tone, "Go potty!" When he goes, let him get started first, then say, "Good potty!" Using the same quiet, happy tone you use for praise.
I have a very food driven puppy, I can't use food to potty train. He would start to potty then finish as he jumped on me looking for his treat! Praise works for him for the potty deed.
Thanks for taking a GSD that nobody wanted! Good luck to you!
Hondo Von Dopplet L Bauernhof "Hondo"- GSD
Lilie's Tug McGraw "Tug" - Golden Retriever
Maggie - Mini Dachshund (Rescue)
Lonestar Pivo - Texas Blue Lacy
Ashe - Barn Cat
Katie / APHA