I have no idea if your grandson is 6 or 40 but if he took your card it is basically a case of identity theft and fraud and both are a police matter. There is no way to really cancel what he has done if he made the purchases with your card through his Apple account. Apple may make refunds on a case-by case basis but you haven't really provided us with much information such as if this was one through his account or yours.
See your purchase history in the App Store or iTunes Store [iOS, computer, or web] - Does not include purchases made by others under Family Sharing; those have to be checked from their accounts.
http://support.apple.com/HT204088
If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement - https://support.apple.com/HT201382 - "Before you contact Apple to request a refund for a charge you don't recognize"...
If neither you nor a family member (if applicable) has a record of this purchase and you only saw it on a statement for a card you alone control, contact your bank or card company about card fraud. Apple does not handle that.
If you now recognize this purchase and are asking for a refund then realize Apple's terms of purchase for most countries are effectively, "All Transactions are final" (check terms at: http://www.apple.com/legal/internet-services/itunes/). Apple does refund on a case by case basis, but they can also refuse.
Request a refund for an App Store or iTunes Store purchase - https://support.apple.com/HT204084
Report a Problem - http://reportaproblem.apple.com - "Sign In for help with iTunes purchases made in the last 90 days"
If a reporting a problem request is unsuccessful you can also try:
Other ways to contact iTunes Support:
https://www.apple.com/emea/support/itunes/contact.html
or
https://support.apple.com/country-selector/itunes