Well, now... I'd say it's typically that mom or dad are the ones "catching the kid in action" and have to deal with smoothing things over.
But, it does happen this way sometimes. There's nothing to feel guilty about. Really.
I'd suggest you take the first opportunity to have a talk with him. But you must make light of the situation. Because, chances are he's known for a long, long time that his old man jacks off. And, apparently it didn't bother him to see you with your cock in your hand; certainly not if he brought you the towels so you could clean up. Hell, you can even laugh about it. That's the best way to let him know, to get him to understand, that no matter how old we are and regardless of our relationship status, men jack off. It's pretty plain and simple.
Plus, what better time of year could this happen than Masturbation Month? It's a great opportunity to let him in on all the reasons we do. Yes, first and foremost is likely to be that we're horny, but tell him about the other myriad reasons. But, truly, just because it feels good, we enjoy it, so why not?
I think it's great that he didn't make a big deal about it. Now it's your turn to make it something normal and common. In fact it's so common there are multi-millions of guys stroking away right at this moment.
Hell, some of them acknowledge it's so common, they're stroking together.