The only thing that you can do, is inform him about sex and the precautions, he must take before engaging in such activities. As for the other children, you son will still read and or watch these videos. I suggest putting a tv, for him only somewhere, the other children are not allowed, do not put it in his room, for it offers to much distraction. Set a time for him to have his alone time, if and only when he does his school work, and let him know that if his GPA drops the tv is going. As a boy with hormones, that are out of control, he will still do these things even if you say no maybe even more so then, so tis better that you know what he does.