Cancelling Microsoft ultimate subscription on xbox

RLH33

My son has managed to set up a few accounts on his Xbox to take advantage of cheaper 30 day deals.  However he didn’t cancel them at the end of their reduced fee period and now has numerous subscriptions draining his account every month.  He only gets pocket money so it’s eating every penny every month.  In a panic he’s cancelled all the accounts, but not the subscriptions, so now we have no account, no login details and no email address for the accounts.  I’ve rung the bank to cancel but they said I need to cancel it with Xbox - but I can’t, every time it asks for the login details.  I’ve gone through the website, online chat and rung them but they cut me off as soon as i said it was for Xbox saying - log into your account.
What do I do? I have cancelled his pocket money , he’s only got £3 left and his account cannot go overdrawn, I’ve cut up his card too.  I’m tempted to ring the Halifax tomorrow and just close his account but they said I didn’t need to do that, I just needed to contact Xbox arrgghhh!  I am so angry with him and just want it sorted but I’m hitting my head against a brick wall - any ideas or advice?

DCFC79

For the login in details use the forgot password option and reset the password and do this for how many accounts he has, he surely knows the email addresses used for each one ?