I accidentally synced up two iPhones I need to undo that

Oh boy. It sounds like you signed in with his Apple ID on your device. When you go to Settings, whose name is listed under the 'Apple ID' pane? If it's his name, you can sign out by clicking [his name] and finding the 'sign out' button. Otherwise, I would contact Apple Support: Get started

When you say you 'synced' two iPhones, what do you mean?