The system is set up so each user always has their own ID. If his ID is signed into your iPhone then according to the system the iPhone belongs to him. Your best course of action will depend upon how amicable you are. Ideally he will do the things in this article so you can make use of the phone again after creating your own account.
This document explains how to create a new Apple ID (create an Apple Account) --> How to create a new Apple ID - Apple Support
Use the steps in this support article to remove your personal information from a device, even if you don't have it anymore. --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
It would be more convenient to you if he omitted to erase the device (but definitely disable Find My), then you would be able to delete his information after signing into the phone with your new ID.
If he has nothing to do with you then you have a bigger problem because the ID signed into a phone is the one that owns the phone. The only option then is to ask Apple to reset the device entirely. You won't have any of your data but at least you will still be able to use the phone.
Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not detail what currently constitutes proof. Prior to this new online service Apple typically required the original receipt issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership). This will give you access to the device but won't restore access to the account used to lock it. There is nothing that can be done to expedite an Activation Lock request.
Apple recommends you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support