Unfortunately hemangiosarcoma is very common in gsds. My friend scheduled the surgery for her rottie before the tumor got to a dangerous point. Her rottie did have a full recovery and is alive and well today. However, the chances of it not being hemangio are very slim.
If it is hemangiosarcoma then even with surgery and chemo he will most likely not live more than 6 months. I know that in at least one case she was very happy she had the surgery and did the chemo because she really enjoyed those last months with her dog. I also remember people on this board who opted to have surgery and their dog only lived a few more days.
It's a very tough decision. When I took Basu in he was very weak and when I was told the chances of him surviving I decided to let him go. It was heartbreaking and very sudden.
Big hugs to your and your boy.