If it were me, I'd change his password.
Even if he changes the password the Administrator can "remove" this password and then access the account with no password.
If it is just files you want to hide, although I don't know why you would, I would just download some kind of encryption tool and encrypt the files you want to keep secret.
You could also keep secret files on a USB drive. You can encrypt the entire USB drive's file system so it takes a password to access the drive in the first place. This is what I use at work to keep server passwords secure.