Had to reset and can’t figure out what email was used but have gamer tag how do we find...

So we had to reset xbox and now we can’t figure out what email and pass word was used to make the xbox account. I have his cod log in linked but it doesn’t give more then the gamer tag. If i unlink can he sigh. in on the new account and not lose all his stuff? or how do we find the email. we told it to keep all game data but it reset the whole thing

:)