He should sign up for his own Apple ID at https://appleid.apple.com/
he will however need a separate email address to sign up and where it asks for a credit card there is an option to not have to put a card in if so inclined
After that he should go to settings>iCloud and log out from your Apple ID then sign in with his new Apple ID
As well go to settings>app and iTunes stores and log out and then sign in with his new Apple id
optimally to really clear out your info from his phone to be sure is a full erase and have him enter his Apple ID but this would erase everything back to factory
go to general>reset>Erase all content and settings. Again everything gets wiped and the phone is set back to factory if you do this
if going this route you can sign up for an Apple ID here on the setup screen when his phone reboots.
this probably happened because iCloud was enabled on your device and he just recently enabled it on his. If you use the same account pretty much all your data will sync on both devices regardless of phone number