Having a father with prostate cancer, I can relate. My dad typically does not share personal matters such as his prostate much either, but this entire experience with cancer has forced us to discuss the topics that are typically off limits for a father and daughter to have.
If you would have told me a year ago, that I would be asking my father about his urine and bowel habits, not to mention, the use of Viagra, I would have never believed you.
It is interesting how everybody reacts different. I ask my dad straightforward questions and he answers. My mom asks him the same questions and he gets mad at her. My mom and I laugh about it. I agree with the above posts regarding confronting your dad about the issue. It's a heavy burden for someone to carry alone. Even if he confides in your mom...it's a lot for your mom to carry.
I'm sure your dad is trying to protect you. That is what dads do...but us daugthers...we worry. :) Good luck and god bless!
Another doting daughter