Other ways to find his Apple ID
He can check to see if he is signed in to the App Store, FaceTime, or Messages on his iPhone, iPad, iPod touch, or Mac, or check to see if he is signed in to iTunes for Windows on a PC. He can also go to iCloud.com or appleid.apple.com and see if his Apple ID prefills on the sign-in screen.
If he forgot the Apple ID (iCloud) password, enter the Apple (iCloud) ID, and click Forgot Password. See if he can get a Rescue or Notification email option if necessary. Link to contact Apple Support at the end of the 2nd article.
Apple ID - Manage
Apple ID – Forgot Password *
If he has 2 Factor Authentication enabled, if he can't recover the password using the steps above, he will need to use Account Recovery. The Recovery Process is an automated process and the length of time it takes can vary a lot. Apple ID Support can only answer your questions. They can't intervene in or speed up the process. The article below has some suggestions to shorten the time the process takes.
Apple ID - Recover your Apple ID with two-factor authentication account recovery. *
If necessary, if you are the original owner, contact Apple. Apple can remove the ID if provided the required paperwork to include the original purchase receipt with the serial number on it.
Request additional support for unlocking your device.
