How do I cancel my son's Xbox live? His xbox is broken and he put in a wrong email address

http://support.xbox.com/en-GB/billing/xbox-subscriptions/how-to-cancel-xbox-subscription should tell you everything you need to know, assuming you or your son can remember the account details (email/gamertag and password) used.

The relevant section is below:

In a browser, sign in to the Services & subscription page with the Microsoft account that is associated with your subscription that you want to cancel.

Note If you don't know your Microsoft account email address or password, use the Lost Account Solution or the Lost Password Solution. Find the Xbox subscription that you want to cancel, and then follow the instructions to cancel.

The best bet is to contact Microsoft directly. Even with the wrong email address, you may tell with your credit card number what account to disable. Be prepared however to provide proof: Microsoft is known for their poor customer service.

They may also provide tips to find the email address linked to the credit card