I think the issue here is going to be more about his mental health than anything else. Being diagnosed with, treated for and recovering from cancer is traumatic - to then be diagnosed with a completely separate cancer must be devastating.
As I think some of us probably said on your previous thread, it is good that he has been considered suitable for active surveillance and the specialist would have been unlikely to recommend AS just because of the previous treatment. If they thought he needed treatment now, they would have banged him onto hormone therapy before you could blink. As long as AS is done properly, it should be fine - regular PSA tests, regular DRE, regular mpMRI scan and repeat of the biopsy if there are concerns that the cancer is becoming more active. The skill of the oncologist will be identifying when to start HT.