Hi. I'm not an expert,and it's a very complicated scenario, and its quite possible that no one on here has been faced with exactly the same as your dad.
I think the situation will only become clear after your discussions with the oncologist, who will give you a proper treatment plan, plus his reasons.
When I was at your stage, just.being aware that I had cancer, but not knowing if it was virulant and if it had spread, I was comforted by the knowledge that we could be treated by hormones which might well control the cancer wherever it had spread. Many men survive for years with it being controlled, yet not cured.
Try not to keep analysing, it'll drive you up the wall, and whatever you think, you will almost certainly be wrong.
Try to be positive and optimistic