I'm not a parent but rather a 19-year old son. Unfortunately, I too was caught doing the deed or whatever metaphor you choose to use by my father. I'm sure he knew, in fact, I'm certain he knew but he pretended like he didn't. For me it was internet porn and I never heard him enter the room. Anyways, he didn't really care then but later we had the infamous "sex talk". I have no idea if he told my mother and I never felt the need to ask, not then, not now and not in the future.

There are programs to download to try to minimize the amount of porn and such he can see on the computer, however, these programs are not perfect. For TV, you can set a parental control for certain TV/movie ratings. However, remember that at that age and in this era, he'll still get his hands on it sooner or later.

You can talk with him about it, let him know you've been there, done that, etc... . Telling the mother is up to you. He may be embarassed to talk or even deny it that he was doing the deed.