One Apple ID being used for multiple phones. How to create a new Apple ID for one?
Go to http://appleid.apple.com and grab a new Apple ID.
Open the Settings.app on your dad's iPhone.
Select "iCloud" and scroll to the bottom of the page.
Select "Delete Account".

Don't worry, this won't delete the actual Apple ID account, it will just remove it from the phone.
Now login to the new Apple ID.