Good day, Apple Community support!

If it is activation locked to his AppleID password, only he can release it -> How to remove Activation Lock - Apple Support

There is nothing you can do and Apple will not assist you. It is up to the previous owner to release it.

For the Apple ID password, have him enter the Apple (iCloud) ID and click Forgot Password.

Apple ID - Manage            

Apple ID – Forgot Password          

If he can recover the password, have him do the steps in the article below to make it 'your' phone.

Selling/Giving Away Device.      

If he can't do the above, he needs to contact Apple to have them reset the phone or it will be worthless to you.  If he is  the original owner, have him contact Apple. Apple can remove the Activation Lock if provided the required paperwork. Either take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store (if it has reopened)

Genius Bar Reservation and Apple Support Options - Apple.    

or contact Apple ID Support. 

 Apple ID - Official Apple Support.    

Contact Apple for help with Apple ID account security - Apple Support.  (Link doesn’t always load)   

Get Support      Work through the screens to get a chat option. 

Apple Support schedule phone call

https://twitter.com/AppleSupport       People have posted it may take an hour or more to get a response.