Using New Outlook on Windows for personal email, calendar, and contact management
Hello
Are you saying your son removed your hotmail.com address as alias, and you got that error when you tried to add it back? If so, I hate to tell you, once an alias that is outlook.com or hotmail.com is deleted, it is gone. It cannot be added back or recovered.
Microsoft explains it here: https://support.microsoft.com/office/add-or-remove-an-email-alias-in-outlook-com-459b1989-356d-40fa-a689-8f285b13f1f2
Under the section "Remove an email address", it says: "Are you sure? If you remove an alias that's an email address from a Microsoft domain (like @hotmail.com, @live.com, @outlook.com, or @msn.com), you're permanently deleting the alias and it can't be associated with any Microsoft account again."