What does your father want to do? At 90, I'd check into a hospice , knowing what I know now about how the treatment changes your quality of life especially if my surgeon said I might not survive the surgery etc. I am 65 and have been fighting this for 5 years, and at my age, it is very doable. But 25 years from now would be a different story.
So sorry that you have to deal with this. Don't worry though, any decision you make will be made out of love and in the end that is all that matters.
Charm