If it is an American credit card, it is easy to call your credit card company and request a “charge-back” (also called a dispute). Claim the charges were unauthorized. Tell them charges agreed to were different (and explain.) If your son is not on your credit card as an “authorized user”, also claim the contract was not signed by yourself or an authorized cardholder. If he is an authorized cardholder, claim the signed agreement does not allow the merchant to make those extra charges.