When someone asks a question I can't answer I will often start Googling for studies, ostensibly I am looking for answers to the question, but actually it is just something to do while I wait for Tall Allen (or one of the other smart guys) to show up. But, since Tall seems to be a little slow responding to the bat signal this time, here's what I found.
There have been two big recent studies that almost apply to your dad's situation -- almost but not quite.
The first is
Efficacy and Safety of Abiraterone Acetate in Elderly (≥75 Years) Chemotherapy-Naïve Patients with Metastatic Castration-Resistant Prostate Cancer that looked at adding Abiraterone acetate (Zytiga) to the ADT for elderly men with castrate-resistant disease. They divided patients into two groups by age < 75 vs ≥75 Years. They found a nice benefit for both age groups and the older guys seemed to tolerate the Zytiga about
as well as the younger one, basically pretty well. But that study was for castrate resistant disease which your dad doesn't have... yet, anyway.
The second study --
Adding abiraterone to androgen deprivation therapy in men with metastatic hormone-sensitive prostate cancer: A systematic review and meta-analysis looked at results for treating hormone-sensitive cancer (like you dad appears to have) with Zytiga. Again there was a pretty clear survival and QOL benefit and the treatment was well tolerated but nobody in the study was as old as your dad.
So if you put both studies in a jar and shake it up you get no compelling reason why your dad shouldn't try Zytiga and Prednisone. It's usually well tolerated and might give him a few more comfortable months. But it costs money and it will give him almost the same benefit if you wait for him to become castrate resistant and then start the Zytiga.