Holly, I've just come from my first chemo session this afternoon. It was a piece of cake and so it should have been. This is now a routine procedure offered early to advanced PCa cases. In my case though, after nearly ten years of other treatments. I have Mets outside the local area in lymph nodes so a bit like like your Dad. A bit younger than him at 73, but probably with a similar outlook. You really must not worry or fear death is near. It isn't. After the course of chemotherapy treatment, if other treatments are needed, there is an increasing choice.
Regarding life style choices and diet, just encourage a healthy balanced diet, forget supplements and fads, eat the things and do the things which are good for the heart. Gentle exercise, nothing strenuous, dog walking a few miles a day is great.
Use your times with him to boost his confidence and think positively about the future. I suspect he may surprise you with his longevity!
AC in Leics.