We are fellow users here on these forums, we won't know what it's for. Did your son make an in-app purchase in the app and/or did you have any auto-renewing subscriptions that were due ?
You can view the purchase history on your computer's iTunes : log into your account on your computer's iTunes via the Store > View Account menu option, you should then see a Purchase History section with a 'see all' link to the right of it. Click on that and you should then see a list of your purchases
If neither you nor your son (nor anybody else who has access to your acocunt) made the purchase then try the 'report a problem' page to contact iTunes Support : http://reportaproblem.apple.com
If the 'report a problem' link doesn't work then you can try contacting iTunes support via this page : http://www.apple.com/support/itunes/contact/- click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption