My GSD puppy limps after injury, x-rays clear. How to help him?

If the radiographs were not remarkable, the pain Hugo is experiencing is likely from soft tissue and / or nerve pain. True ligamentous or tendinous ruptures do not often occur in the forelimb in the dog, though it is possible. Advanced imaging or scoping would be needed to confirm a diagnosis. We tend to see these issues in the hind end most often particularly in larger breed dogs like GSDs. The best things you can do for Hugo right now are the following:
-Most important thing: he needs to rest his leg; that means no going outside unless on a leash, and even with leash, only to do his business. Should limit use of the leg as much as possible over the next 5 to 7 days. This will be difficult with GSD puppy but it is needed. If your veterinarian did not discuss the possibility of using a small sedative over the course of the next week, you might bring it up should it become too difficult to keep Hugo down.
-Give any prescribed medications from your veterinarian as directed. If you walked into my office, I would have prescribed an anti-inflammatory and some pain medication. Hopefully your vet did the same. If he did not and Hugo is still experiencing discomfort you should call and request meds.
Hope this makes sense. Consult with me if you have further concerns.