Hi, I'm John! I will help you with this.
Is it possible that the account has been hacked?
If you’ve already confirmed that the email address was entered correctly and you’re still seeing the message that the account doesn’t exist, it’s possible that the account may have been compromised.
To help narrow down the issue:
Do you or your son remember receiving any security alert before this happened?
Also, please try searching for the account using these steps from Microsoft: https://support.microsoft.com/account-billing/forgotten-your-microsoft-account-username-b2049472-3b8f-27d3-61c6-67a668453f4c Click on "Search" and follow the instructions provided.
If you receive any error messages, such as “account doesn’t exist” or anything else, please let me know.
Click "Add comment" to let me know.
Regards,
John J.D.