I forgot my email address for my Microsoft account but I can remember the gamertag and...

If it is an account on a Xbox One console, the account is tied to a Microsoft Account email.

If your son still remembers the email and password to that account, they can simply re-download their accounts with that sign in information.

However, they don't sign in with the Gamertag name, but with the Microsoft Account email address.

Should your son have trouble remembering the email address, he can
follow the instructions here to try to recover the email.

We recommend checking this link Find your Microsoft account email address | Xbox Support

It sounds like you created a new account instead of signing into the old one. You would need the correct log in information for the original account.

If you can't remember the Microsoft account, try this https://support.xbox.com/my-account/microsoft-account/forgot-microsoft-account-solution
Once you have the email, you can reset the password if you do not remember the correct password.

that is not what happened...i´ve always used the exact same e-mail address and now my e mail address has a different gamertag. I have never created "CastGosling2133" It was just there instead of my real gamertag...

Hello, My name is Jesse. I'm an Independent Advisor and a member of the Microsoft community and I'd love to assist you today.

I'm afraid only way to recover an Xbox account is via the account recovery form here https://account.live.com/password/reset

You can try all the emails you remember or if you had a phone number attached to the account, you can use that too.

Hey there, a lot of the time this happens you have actually used the wrong email address as I believe it's impossible to have two gamertags on the one email.

To find the correct email address if you can still sign-in to the profile on the console through the auto sign-in feature you can see the email in the top right corner of the screen:

support.xbox.com/.../forgot-microsoft-account