What do I do if my son used my card to make online purchases for a game or something but I didn’t approve of it. What do I do

If you gave access to your son to use your card that is your responsibility. You first need to revoke that access. You can request a refund by logging in to https://reportaproblem.apple.com, where you can request refunds. Apple is pretty reasonable about refunds for mistaken charges, although they do say that all purchases are final.

You may please read this artivle --> If Ask to Buy isn't working - Apple Support