So sorry to hear your awful news, you must be devastated. I had a friend with a GSD who had a tumor on his nose. it was a sore which didnt heal. It took a long time for the end to come and when it did it was peacefully in his sleep. His quality of life was excellent even at the end. He just got slower and sleepier.
When I lost my beloved lab, Pepper, to toxic shock after being spayed it was quick and with no warning. I would have given anything to have known those were going to be her last days. I would have made loads of vid and took loads of pics of her, and made a huge fuss of her. As it was she went to the vets for observation and never came back. I never even had a chance to say goodbye.
Try to take some comfort from the fact that you will at least have that with your little fellow. Spoil him and enjoy the time you have left together. It's devastating and heartbreaking, but also a precious and momentous time in your relationship. Savour every minute left and then you will have wonderful memories to look back on.
I hope I havent made you feel worse
big hugs and kisses. Again, so sorry.