I am trying to retrieve the Apple ID for my son’s iPad mini 4

Typically an Apple ID is an email address you have used. If you are currently using a mobile device that is logged into your account, go to Settings and tap your name at the top. Look in Keychain, and in a browser's security settings if you use it to automatically sign into Apple when using a browser.

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/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