To hide apps he must be using a launcher... Does your home screen look different... It means he is using a launcher.. Done these launchers help people apps... In this case u can go settings/apps/downloaded apps/ and here u can search for any launcher apps.. Click on it and uninstall it.... And once u return to your homescreen go to the app drawer and u will now be able to find hidden apps(if any)... He might also be using an app like parralel space which allows installation of other apps inside this app.. In this case the installed apps would not be seen in the usual app drawer. But only inside this app... He could have protected this app with a password... That's all I can suggest u do