Yes, however, you shouldn't word it in such a way that it sounds like you're going to be big brother.
You should however set up a bunch of front line defenses. Get some internet filtering, make sure he's not running the computer as an administrative account, etc. Windows does have parental locks for standard accounts as well. And where you go from there is up to you.
EDIT: Actually you should probably just educate your son more about internet safety and being weary of strangers online and set up privacy options so some creeper can't see his account.
Although everyone has the right to know the truth, children are too young to understand what their parents monitoring them and just think that you do not want them do scan some websites. The more you restrict them, the more curious they are to watch those webs you want to be banned.
So in my opinion, you can tell them what you really did until they have grown up to understand you.