Do you mean the Settings > General > Restrictions passcode ? If you do then you can change it via Settings > General > Restrictions > Disable Restrictions, type in the passcode, and that button should then change to Enable Restrictions, and tapping that will allow you to set a new Restrictions code.
Depending upon what version of iOS that you have on your iPad you may also have a Require Password setting on the Restrictions screen which you can set to 'Immediately'.
If he also knows the password to your iTunes account then you can change that, either directly on the iPad by tapping on your account in Settings > Store (Settings > iTunes & App Stores on iOS 6) and tapping View Apple ID on the popup and logging into your account, and tapping on 'Apple ID' on your account screen ; or you can do it the Store > View Account menu option on your computer's iTunes ; or by logging into your account on https://appleid.apple.com/
You could also try contacting iTunes Support and see if they will credit or refund you : 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