Gamertag Email address

It sounds like it wasn't an alias created but a completely new Outlook.com user account?

If that's the case, I don't think you can sync the achievement's and game data over

changing alias can take a few days to synch up, log in with the old email address for the time being until it takes hold, have heard it can take a couple of weeks.

Hey there!

Thanks for the support message.

When you changes the sign in email for the account, you will need to remove and re-add the account to the console. Please double tap the Xbox button on your controller and go to Settings>All Settings>Account>Remove accounts>Remove the account you changed
the email for. Then power cycle your console by holding down the power button on the front of the console for about 15 seconds until it shuts down. Once done, double tap the Xbox button on your controller, go to the profile tab at the top>Add "& Manage>Add
new. Type in the new Microsoft account email address and password for the account.

Cheers!

Can what be retrieved ?

Have you tried logging into your sons account ?

Hello there!

In order to know your e-mail address that is linked with your GamerTag please try xhecking the steps on this link Find your Microsoft account email address | Xbox Support.

If the issue persists please try contacting support for more help Contact us | Xbox Support.