Hey Tezzle82,
Sorry for the frustration you're running into just trying to get your son online here.
Unfortunately, I don't know why you would have been referred to the forums for an issue like this one. We are unable to discuss account details or make any changes at all to accounts via the forums, as these are public boards visible to everyone.
If these steps aren't working to age up the account and you're unable to log in due to incorrect credentials, our support team would need to take another look at this with you.
Just to clarify, resetting your Xbox to factory settings shouldn't have any impact on the actual account(s) being used there when it comes to registered information like a date of birth, so that should be unnecessary. Each EA account can only be linked to a single account from each platform (Xbox), so if you were to start a different Xbox account you likely wouldn't be able to link it to the same EA account with your son's current progress and data.
Ideally in this case our support would be able to help verify and update any incorrect account details, rather than trying to unlink or move around different accounts altogether. You should be able to resume a previous case from your case history page here to pick back up with an advisor as needed.