Mac OS X 10.6 Parental control question

I need to add some parental controls to my imac, there are two accounts set up now. One for me and one for him. I need to know if I go into his account and set up the control sso he cannot access certain sites what is going to stop him from going into the preferences and changing or altering the settings?

Is there a way to set the control prefs and any changes made can only be done with an admin password which he will not know or make the parental changes from my account and set them for his as he does not know my password.

??

R