cant get onto game account

nothing you can do, as the account is in fact his email account then there is no way for you to get access, the only thing you can do is create a new account.

a microsoft account belongs to the person who opened it and uses it and is registered too.

yes that's actually possible, you can do this, this video will explain it better that me https://www.youtube.com/watch?v=cLSjj40qUgw.

unfortunately, unless your ex-partner can and is willing change the email address on the account (only possible if it isn't a microsoft email address) then that account is gone.

as you will have to start a new account for your son I recommend that you give him his own email address and set up the account as a child account then create an adult account for yourself (unless you already have one) then create a family group and add
your son to it.

this link has all the information on how to set up your Family Group and manage the Parental Controls for Child accounts

https://support.microsoft.com/en-us/help/12413/microsoft-account-what-is-family-group

this will make life easier in the future as you will be able to set up privacy and online safety for him as well as controlling what types of games he plays and for how long.

it will also mean that when he's older his Xbox account will be on his email account and not yours

Click the second link in my previous post and reach out to phone support.

From what BleedingFerret said I'm guessing whoever was using your GT got you banned then switched the Email that was associated with killagamea1341 with thetemptingrelic28. And since you still know the email and password you got the thetemptingrelic28
GT because that GT is linked to your E-mail address.