If he is currently also using another mobile device that is logged into his account, go to Settings and tap his name at the top.
This support article describes ways to find your Apple ID --> If you forgot your Apple ID email address or phone number - Apple Support
or go directly to this link --> https://iforgot.apple.com/password/verify/appleid#
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."
Contact Apple for help with Apple ID account security - Apple Support
Also look in Keychain, and in a browser's security settings if he uses one to automatically sign into Apple when using a browser.