Um, masturbation is a pretty natural thing. I never understand why people these days are horrified of sex. I'd have been more horrified if your son had accidentally seen the movie Saw, to be honest!! So he saw your husband doing something he probably doesn't even understand. Maybe he's tried it out himself now. So what? it's just masturbation. I'm pretty sure that YOU are the traumatized one, not your son.
My advice? Talk to your son. Ask him if he saw daddy on one of the tapes when he was making copies - for all you know, he didn't! If he did, then calmly explain to your son what your husband was doing, and that it is 100% natural and it's okay if he does it too. Tell him that this is an activity that should be done in the privacy of the bedroom or bathroom, and that people don't normally tape it, but daddy did it as a special gift between married adults.
If your son asks more questions, be prepared to answer them honestly. Don't lie. That's the most harmful thing you can do to your son. If this discussion leads to questions about sex, then tell him. You can repeat kindly that these are activities done between two married or loving adults to maintain the idea that he as a child should not be doing them yet.
Your son is 8. I knew about how babies were made when I was SIX, so I think your son is LONG overdue for "the talk". I asked, so I was told. In this situation, your son has seen some sexual behavior and now he needs an adult to explain this behavior to him. He isn't going to understand otherwise. He may try to masturbate in public. He's not perverted or traumatized or sick - he just doesn't know how to appropriately understand it.
So please, go talk to your son. He is not damaged. He's just fine! You're the only one freaking out, I assure you.