recover hacked Gamertag

My son had an email associated with his Gamertag for 10+ years. Recently someone hacked into the internet service provider email account associated with the Gamertag. The hacker forwarded our emails to their email address. That person must have requested a password reset for our xbox, but since he had forwarded our emails to him, we never saw the requests. The password was reset, then the email we had associated with that Gamertag was changed to his email. My son can turn on his xbox, see his Gamertag but with the different email address. When he tries to log on with his email, a system message says there is no such account with his email. Is there anyway to essentially get his account back? We have tried to contact Microsoft through a link but the response is we don't give them enough information. We've tried the Chat and that keeps sending me to reset my password, which is not what we want. I've tried the 800# but can't get a live person. The automated system then gives me a link to change the password. Again, what I'm trying to do is get our email address linked to his Gamertag so he can access his account. Thank you in advance.

:)