Instructions
Call Microsoft's Xbox support line at (800) 4MY-Xbox, if you wish to talk to an Xbox support specialist over the phone. Have your son's gamertag as well as the Windows Live ID associated with the console. You also need the last four numbers of the credit card on file as well as the first and last name of the person the account is registered to.
Go to the support section at forums.Xbox.com. Click "My LIVE account was suspended." Create a post on this forum asking for clarification on the gamertag ban. This forum is staffed by Xbox Live support specialists who can access the account information and let you know the length of the ban and the reason for it.
Email Xbox support through the "Contact Us" link at support.Xbox.com. Click "Email Us" then "Xbox Live." Enter your full name and email address. Choose "Gamertag question" from the drop-down menu and describe the issue with your son's gamertag. Click "Continue" to send the email.