Greetings!
This is Tin, I'll do my best to help. Sorry to hear what happened. No worries, you can remove your card from your son's Xbox account and you may also try to request for a refund for unauthorized purchases.
Please go to:
https://account.microsoft.com/billing/payments
Sign in your account and remove your card from there. If it's not showing, ask him to sign in instead.
You may also try to request a refund from this link:
https://support.xbox.com/refund
Thank you,
Tin