When you set up family sharing, the family organizer agrees to pay for all purchases. That’s just the way that Apple set it up. He can still use his card to make online purchases with other merchants and so on, but if you are the organizer you must pay for all app, movie, books, music, etc purchases.
There is a very easy way around this. Have your son redeem Apple gift cards to his Apple ID balance. As long as he has a balance from the cards in his account, your credit card will never be hit as long as he doesn’t exceed his balance. His account will be used for purchases before your card is charged. My family has been paying for their own purchases with Apple gift cards since day one. I am the organizer and the only time my card was hit was when my daughter used up her account balance.
The text that I pasted below is from this kb article.
Family purchases and payments - Apple Support
Which payment method is charged for family members' purchases?
One adult in the family—the family organizer—pays for most purchases that family members make. But if you're a family member and you have your own Apple ID balance, your purchases bill to your personal Apple ID balance. Some purchases, including gifts, can't be billed to Apple ID balance and will be charged to the family organizer's payment method.