You need to set up Family Sharing and then use the Family Sharing Apple ID link above to create an ID for him.
Family Sharing
Family Sharing - Set up
You can sign into the phone using your Apple ID to set up Family Sharing and create the ID, then sign out of your ID, and in with his ID. Use the article below as a guide of where to sign out/in. If you need to do anything else for Family Sharing, you will have to sign him out and you sign in to take care of Family Sharing details, then revert back to his ID.
What to do after you change your Apple ID or password - Apple Support