Hi,
I was diagnosed at stage 4, 4 years ago and often have similar thoughts. Physical activity and a healthy diet will certainly help in part because they help keep a positive mindset. There will be loads of specialist diets and foods that are advertised on the internet - but I am sceptical of the evidence supporting most of the claims and I've never been any good at sticking to diets.
I think there are two ways of looking at the survival issue. One is to ignore the stats. If your dad is generally doing well and his PSA remains undetectable, then be thankful and plan to spend time with him doing things that you both enjoy and can continue to enjoy for years to come.
The other is to look at the most up to date statistics. For example if your dad had hormone sensitive metastatic PC and was/is treated with one of the latest hormone drugs (e.g. enzalutamide, abiraterone, daralutamide, apalutamide) - then this link provides the latest survival statistics from the ARCHES trial. https://pubmed.ncbi.nlm.nih.gov/41633900/
Personally, I find some reassurance in the statistics that I might have a few more years in me yet. Plus there are others on this site who have been stage 4 survivors for much longer.
So there is plenty to hope for and lots of reasons to make the most of now.
Best wishes to you and your dad