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
You can also turn off in-app purchases on your iPad via Settings > General > Restrictions > In-App Purchases 'off', and you might also have a Require Password setting on the Restrictions screen which you can set to 'Immediately' (whether that setting will appear will depend upon what version of iOS that you have on the iPad)
It's a bit late now, but before letting your son use your iPad, you should've done this:
Settings > General > Restrictions > Enable Restrictions > Enter a PIN
Settings > General > Restrictions > In-App Purchases > OFF
Settings > General > Restrictions > Require Password > Immediately
This is all in the User Guide, of course.