Boot into safe mode and see if you can log in as Administrator. (Hopefully he hasn't specified an admin password or if he has he remembers it). Once in there, you can create another account and use it to log on, or you can remove the password using the Users applet in the Control Panel.
If there is an administrator password and you can't remember it, then you'll have to download a password removal utility... I can't remember where I found one, but just Google "Lost Administrator password" and you'll be able to find it easily enough.