Having had similar probs with mine, I offer the following: change your son's settings to 'child' on NAV, that will stop him going on 18+ sites. Make sure that he doesn't get to log onto your login though.
Think back to your own youth for a moment. Wouldn't you have wanted to look at these sites when you were his age? A big part of adolescence is rebellion - especially rebellion against one's parents.
The more you try to prevent your son seeking out these sites, the more determined he will be to look at them. This could be on your (his) computer or on someone else's. You are wasting your time trying to prevent it. As for "nanny" software it either doesn't work properly or you will find that a popular subject of conversation at schools and colleges is methods of over-riding the various nanny programs.
You would be far better off in giving lessons in sensible Internet use and arming your computer with as much anti-malware software as is practical.