The issue is whether or not you can actually have a binding contract with someone under the age of 18. Usually the answer is no, but it sometimes depends on the circumstances. It is impossible to tell from the facts given.
As previously stated, you can't have a binding contract with a 17-year old. As a result, there is no contract. You will likely need to refund the money (and he'll have to return the go-kart).
Any response is not legal advice and should not be considered legal advice. Always consult an attorney in your state or jurisdiction, as laws vary from state to state.