Assuming you are the main NTL account holder, go to
http://www.ntlworld.com/helpsupport/...ail_manage.php
and click on 'Add extra mailboxes'
Put your email and password in the next screen and press continue.
This will bring up a list of mailboxes associated with your account - and should tell you your sons address.
Then you should be able to retrieve the password.
(Probably need to be on an NTL IP, not at work)
HTH