Using older versions of Outlook for Mac in business environments
Steve H, I understand how frustrating this can be but there is nothing more you can do as the alias cannot be recovered.
If he deleted the alias, it cannot be recovered. I pulled this information from Microsoft article: https://support.microsoft.com/office/459b1989-356d-40fa-a689-8f285b13f1f2
"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."