I think the others are on the right track wiht their advice.
I keep our home puter in the play room, i alwys can see it and who is on it. We have the parental block up for the puter, and even you tube has a parental block! Use them, that is what they are there for. SO we got that figured out, but do you think he is ever gonna forget the website address? Nope, and when he is at his friends house, what do you think he might do? You may want to address this with him and set some boundries......Know your kids friends, and their parents, call parents and make sure they know what he is allowed to do and not allowed to do. This is your job and theirs if they want their kids to have friends over. Tell him he needs to be trusted. (I sense that since HE KNEW ALREADY IT WAS WRONG, that is why he turned the puter off) There should be consequenses for this, not just a talk in my opinion. Good luck!!
