Hi Phil.Surf 16,
Re: "Help With Forgotten ID"
Wondering if the user ID is still your own Apple ID,
or
whether it was changed to an Apple ID for your son.
The iPad's Apple ID may be in use with iCloud, Messages, FaceTime, etc. and is generally an eMail address (in a few countries it can be a phone. number).
The iPad may also be already signed in with the Apple ID:
How to: Sign in with your Apple ID - Apple Support
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Here's the link to Apple ID online log-in page:
https://appleid.apple.com
How to: Get a verification code and sign in with two-factor authentication - Apple Support
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Else: Apple support has help / articles online that may help:
What to do: If you forgot your Apple ID email address or phone number - Apple Support
or
Enter information to find your Apple ID:
https://iforgot.apple.com/appleid
and
What to do: If you forgot your Apple ID password – Apple Support
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Re: "I’m unable to work out how to help him get signed back in as a trusted user".
If needed: How to: Change your Apple ID - Apple Support
Apple Support's article: "Two-factor authentication for Apple ID"
Eg: says:
"Update your trusted phone number on your Apple ID account page
Go to appleid.apple.com.
Sign in with your Apple ID.
Go to the Security section and click Edit.
If you want to add a phone number, click "Add a Trusted Phone Number" and enter the phone number.
Choose to verify the number with a text message or automated phone call, and click Continue.
To remove a trusted phone number, click the X icon next to the phone number you want to remove".
The Complete article: Two-factor authentication for Apple ID - Apple Support
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More info: Apple ID support / help website
Apple ID - Official Apple Support
All the best :-)