Sorry to hear about the cancer - it's a very scary event for someone. How well do you get on with him? Would it be easy to give it to him straight?
A keto-style diet would definitely not hurt, but going from a blissfully ignorant diet, to a keto one is going to be very tough, especially if he is not open to advice from "non-doctors". Intermittent fasting is the most important step in my opinion, try to get him to push breakfast back and exercise gently before his first meal. I know he is quite old (don't mean this offensively) but exercise will help with cancer, not hinder it.
Good luck with this, you have my well-wishes.