To approaches:
1) As computer administrator and owner it will require his information to log in to perform administrator duties. If he has forgotten his Apple ID and password, see this;
Click on this link to read what to do "If you forgot your Apple ID" --> https://support.apple.com/HT201354
Click on this link to read what to do "If you forgot your Apple ID password" --> https://support.apple.com/HT201487
or go directly to this link --> http://iforgot.apple.com/
Also read about "What to do after [or before] you change your Apple ID email address or password" - https://support.apple.com/HT204071
If you still need help:
Contact Apple for help with Apple ID account security "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues."
https://support.apple.com/HT204169
2) If he truly is turning over ownership to you, ideally all this should have been done:
What to do before selling or giving away your Mac - http://support.apple.com/HT201065