What is his prostate cancer diagnosis?
What problems is the prostate cancer giving?
There is a treatment regime called Watchful Waiting, where no treatment is given for the cancer, but any significant side effects can be treated.
There is also a strategy of not doing any treatment until a significant side effect develops.
You need to consider what benefit he might get from treatment, verses how much it might confuse him with procedures and extra side effects which he isn't going to understand, and also how treatment (such as hormone therapy) might speed up the Altzheimers, and extend life after he's lost all quality of life.