It sounds like Microsoft has locked the account for some reason. If your console has been tampered with in any way, chipped to play copied games is the main alteration, then Microsoft will find out via Live with their regular checks that they are allowed to do on your console as per the terms and conditions of Live.
If your son has been obnoxious or rude over voice communication or via the message system and people have made complaints then that could be the reason. it happens a lot more than you'd expect. If his profile had something inappropriate in it then that could also be the problem.
Contact Microsoft support via phone for more information, their email support is next to useless and will take longer than the average ice age to re replied to. You'll find the number here:
http://www.xbox.com/en-GB/support/