|If you have any account that is administrator you can change it back. If he was able to change it then he was logged in as an administrator. Unless he was using your account when he changed the status then you only need to use the same account to change it back. He should only be using a regular account normally and you should keep the administrator account password from him. If he is responsible enough (possibly not) to have an admin password, then he should have his own, but not to use daily.|
If he used the only admin account to lower its status to normal user (not responsible) then you will need to activate your (hidden) high level administrator account. I understand that this is easier in Pro version through managing accounts, but is still possible in Home Premium through command line entry. For more details just use the search on this page or Microsoft's website to search "activating high level administrator account in Windows 7". When you activate this account, use it to fix the other accounts and write down and carefully put this PW away in case it is ever needed again.
You have to be a little bit crazy to keep you from going insane.