What an adorable puppy you have! I can see why you "had" to have him.
Unfortunately, I do have experience with this, though it was a wolfhound and not a GSD, and only one testicle was retained. The issues are similar enough that I'll talk about what I did in our case, in hopes that you find it helpful. First thing is to consult with a very good surgeon with experience in this kind of thing as well as experience in tacking the puppy's intestines to his stomach wall. (More about this in a second). I spent some time looking for a vet with a lot of surgical experience, especially in gastoplexies. I was and remain very happy with the choice I made. I then scheduled an initial consultation with her (as it happens) so that she could examine my puppy (who was about 4/5 months old at the time) and make recommendations.
Based on that consultation, I decided to wait until my puppy was 18 months old to do the surgery. I wanted him to get as much benefit from the descended testicle as possible (e.g., w/re growth plates, etc.) and I wanted him to be as healthy and strong, going into surgery, as possible. As others have noted, cancer is a risk if you don't do the surgery. But it (generally) doesn't appear until dogs are older, so you have the time. Also, last time I checked the percentage of dogs with undescended testicles who develop cancer is fairly small --- though, if it's your dog, it feels like it's 100 percent. YMMV.
Another consideration is whether you want the vet to perform prophylactic bloat surgery (gastoplexy) at the same time. Wolfhounds have a very high incidence of bloat as do GSDs --- though to a lesser extent, as I understand it. My hound came from lines with an uncomfortably (to me) high bloat incidence, so I decided to have both done. Mind you, the gastoplexy (sp) doesn't prevent bloat or GDV, it simply lessens the likelihood of a 'twist.' It also adds to the cost of surgery and to the length of recovery, so keep that in mind as you think about things.
So, off Manny went to the vet at 18 months of age. He was fully neutered, the "missing" testicle located (way up in his abdomen) removed, and he had what I called a "tummy tuck." He came through surgery fine and and made a full recovery.
It's good that you're asking now. That way, you'll have lots of information to hand when you make your decision. Best of luck to you and that adorable puppy.