There something called Parental Control in Control Panel,
Parental Control allows you as the admin to change the allowed hours on the computer, if you want full control of your son's computer change your son's account under
- Control Panel
- All Control Panel Items
- User Accounts
- Manage Accounts
- Change an Account
- Change Account Type
- Change it to Standard
If you really want to catch him in the act, use remote desktop. All you need is a notebook or desktop, them you only need to know his IP, log in to your router to find the info.
Once you got the IP and port, log into remote desktop.