Some great suggestions for the son who walked in on his dad. As a few readers pointed out, it's typically the other way round; it's a parent walking in on a son.
I told the guy he needs tell his mom to go shopping so he could have a man-to-man talk with his father to clear the air. That something so basic to male behavior should embarrass a father is sad, really. I suggested he tell his father that it's great to know he still enjoys himself like that. He might even tell his dad that the activity is "out of the closet" so to speak, that men - as so many readers pointed out - are open about doing it.
The advent of the internet information highway has helped to bring that about.
Of course, he needs to promise not to tell his mom what the issue is (was). Because I suspect his mom has never witnessed her husband jacking off, and it's something his father wouldn't want her to know about. Which is too bad, really. If he could be open about it, they both might learn something.
We need our "me time" regardless of how much partner sex we get.