Cancer isn't a symptom, it's a disease. The symptom is pain. With osteosarcoma, it's excruciating pain, which our dogs will do their very best to hide from us.
I am so sorry for your loss. As always, you gave him the very best possible life. The others must be so happy to see him, and you know they're all waiting together for you to join them on some (very) distant day. Sending love.