How to change administrator/user account please?

I have a 14 year old son who loves playing games. His grades went down last year because of this. So this year we limited his game time and he's been doing fine in school. However, recently, he asked to do his homework online and sometimes if we are not watching, he would open his game to play. I'm afraid this may affect his grades again. So can somebody teach me how to change the user/administrator account? I have a Dell Inspiron 6000. A friend taught me how to add password to the accounts so he has to ask for us to log him in to use the computer. His program is downloaded on the computer so he can play anytime if he has access to the computer. I didn't know the accounts difference so we have just four separate accounts with my passwords and all of them are administrator's accounts. So if someone can teach me how to change his account into a user account, I'd really appreciate it. Thanks a lot.