See the Apple Support article(s) below for IOS 15.2 or later..
Passcode - How to reset your iPhone, iPad, or iPod touch when you forgot your passcode (IOS 15.2 or later). *
ASC YouTube for how to restore your iPhone if you forgot your passcode - Apple Support. -
See the Apple Support article(s) below for earlier than IOS 15.2 .
Passcode - If you forgot the passcode for your iPhone or your device is disabled. *
Change your Apple ID email address and password and do not give him the changes.
Note the restrictions in the article.
Apple ID Name Change. *
Make sure you turn off Find My Phone before signing out of the old ID. Once signed out and Find My Phone is off, then sign in with the new ID, then turn Find My Phone back on.
What to do after you change your Apple ID or password - Apple Support * I suggest doing this before you change your Apple ID or password.
Apple ID Password Change. -
ASC YouTube for How to reset your Apple ID password