Hello,
If his card was added to an Apple ID that you cannot access (i.e. a fraudsters Apple ID) you will not be able to request a refund from Apple directly. You should contact your bank to initiate a dispute and start the process to get new card info for your son so more fraudulant transactions do not occur.
If this was done under an Apple ID you have access to you can request a refund from Apple at reportaproblem.apple.com
Request a refund for apps or content that you bought from Apple - Apple Support
- "If you don't know why you were charged, review your family members' purchases. If you're the family organizer, tap or click the Apple ID button, then choose All. Purchases charged to your shared payment method appear."
I hope this helps.