Is the account that your son is using a limited account?

If you gave an Administrative account then the parental controls will not filter his web sites or create a log of his activity's.

Please go here to learn about user accounts:
http://support.microsoft.com/?scid=kb%3Ben-us%3B279783&x=12&y=14

The article is for XP but applies to Vista as well.

There are parental controls that don`t work with certain browsers (Chrome, Firefox etc). So, if the boy uses let`s say The X Browser and the parental control isn`t compatible with it, you won`t be able to monitor/block. Maybe this is the answer for you...I`m not much of a technical person :)

I recommend you http://www.applicationblocker.com If there are browsers installed on your computer, but doesn`t appear in the blocking list from its menu, you can add it manually.
The easiest way, is to let only one active browser-IE- and to block the usage for others- this way you won`t have problems with both monitoring and blocking processes :)

I use http://www.puresight.com/ without problems, if tou want you can try it for free http://www.puresight.com/parental-control.html