Xbox account/email and gamertag

Like was said above is the best option that you have, you can associate your email to another Microsoft Account but notice, and this is very important, you cannot delete the Main Microsoft account or it will delete everything.

I will try to explain better:

Microsoft Account A - Email A - Xbox Gamertag A

Microsoft Account B - Email B - Xbox Gamertag B

You can associate Email A to Microsoft Account B which will became available to you to login Xbox Gamertag B with Email A by using the Alias system.

But since now the Xbox Gamertag is associated with the Microsoft Account, you cannot delete only that gamertag, you only can delete the Microsoft Account for all (email, account, xbox gamertag...etc).

So if you really want to do this you remember to not delete any of the Microsoft Accounts.

My son has his own account, which is connected to my email...and my other email is strictly work

It sounds as tho the account, son's (gamertag) setup on the XBox is an adult account using your email assigned to your Microsoft account.

What needs to happen is an adult xbox account (think gamertag) needs to be setup with your adult email used for Microsoft account and then use the family setup and make your childs gamertag on the XBox a child account not an adult account.

Your Email account is linked to your Gamertag (Xbox live account) that of which you may be using the wrong email or the account associated with the email may have changed. I would contact the xbox support team via Phone and tell them your real Gamertag and
see which email account it is linked to.

If you signed in and it showed a default gamertag and no score that means the email you used to sign in was not the email on the old account.

With that being said if you are not sure of the email on the old account but know the gamertag you can contact xbox support by going to support.xbox.com/contact-us and give them your old gamertag and if they can verify ownership of the account they can give
you the email thats associated with that gamertag

Hi, when you sign into the xbox change the email on the gamertag that comes up then hopefully when you recover your account again using the same email the second gamertag should show up.

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Steps how to change your email through the xbox here:
support.xbox.com/.../switch-gamertag-to-different-microsoft-account