Can't you put a password on an account? If there's no account he can log in to he couldn't get in, I don't know what you're getting at since you say he has his own account.. of course he'll be able to log in to that and browse..
If you don't want someone to get in your house, don't give them a key and wonder how they keep getting in
Change the password to the account to something you or your parents know. To get in the PC he has to get one of you to log in for him.