Switching email for Xbox but not the Microsoft account ?

Unfortunately this is not possible and since your email is a Microsoft email (Live, Hotmail or Outlook) you can't switch the email to another one and have the account not be connected to that email anymore, technically.

You can switch the login emails around but you CAN'T switch TO a Microsoft email when you do this (Live, Hotmail or Outlook). You can switch to another email provider but you can't switch between Microsoft emails as they're permanently connected to that
account they were created on. For example, you can't switch this account's email login to another Microsoft account because that email is attached to a Microsoft account already.

The only solution is to start over and have him create a new account and have the account that owns all the games to be set as the my home xbox account on the xbox one so that the new account can play all the games that account owns BUT the saves and any
progress will not move over and some DLC will not share, also.

https://support.xbox.com/en-US/games/game-setup/my-home-xbox

Hi there, if you have two gamertags associated with the same email you will need to sign in as one of the gamertags on the Xbox and switch it to a different Microsoft Account (email) using the steps here:

support.xbox.com/.../switch-gamertag-to-different-microsoft-account

Hey, do you know if you have associated both of those gamertags with the one email? If so you might want to switch the email associated to whatever account you can still sign into the xbox with.

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Step on how to change emails here:
support.xbox.com/.../switch-gamertag-to-different-microsoft-account

No, to switch the Microsoft account associated with my gamertag. The email you use to sign in with. I want to switch my email on this gamertag and you are only able to do that on the Xbox 360.

Also, once you have both email addresses, there is a way to switch them to the correct email. Use this a reference ..

Switch a gamertag to another Microsoft account ->
support.xbox.com/.../switch-gamertag-to-different-microsoft-account