How to Block my Teenager from Explicit Chat Rooms

My son has a computer in his room that is connected to the internet.  I have Windows Explorer's security turned up to block explicit web sites, but he can still access chat rooms such as "sexandthecity" and "bachelorette," both of which seem to have plenty of explicit converstations and request for pictures, etc.  

Is there a product I can buy that will block sexually explicit or violent chat rooms?  If not, is there one which will just block all chat rooms?  He is using AOL IM Chat.

Thanks,
Bob