Parental Controls in Norton Internet Security

I am currently running an XP machine with Norton Internet Security 2004. I have a 13 year old son, and he is quite proficient computer-wise. I have installed the parental controls and have blocked any sex-related sites for his user account, and have his MSN messanger conversations logged.

I got up this morning and found that he had logged in at 1:30 a.m., but there was no record of his web history in the activity log section on Norton. I am wondering whether he is able to delete the web history in Norton even though his account is a limited account (set up as teenager, cannot modify his own settings) and his windows account is also a limited account. I realize I am never going to be able to stop ALL his questionable computer actions, but I would at least like to make sure that I know where he's been. Is there a way to block his access to Norton Internet Security so that he can't delete entries in the activity logs?? Generally these reports were my last chance at checking his activity as he regularly deletes his history and temp files. As of right now, he can't have access to the computer until I sort this out. Generally it's pretty harmless stuff, as I've blocked all the nasty content, but I want him to know that he is accountable for where he goes on the internet.

Any help appreciated.