There is a treatment- first a biopsy to confirm, then radical surgery including limb removal, then radiation and chemotherapy, all of which would extend his life for only a few months before the sarcoma would recur. This is so sad, but we have chosen to skip all of it, including the biopsy, and let him live his life as happily peaceful as he can for the grouchy old man that he is.
I hope the tumor grows slowly and lets him live a long time. He doesn't seem to be in pain and it doesn't bother him, so we will see how things go from here. But now, I am kind of sad.