Family sharing of music

Did your son set up Family Sharing to do this? If he did then he is designated as the Family Organizer and if somebody in the family makes a purchase or subscription it will be charged to his credit card unless the family member has enough personal Apple ID balance to cover the cost of their purchase.

If anybody in the family makes a purchase it is first charged to their personal balance. If they don't have enough to cover the charge then the rest is taken from the credit card or whatever payment method the Family Organizer has to ensure payments for the family. It won't be taken from the organizer's personal balance unless it is something the organizer buys.

How apps, content, and subscriptions from Apple are billed - https://support.apple.com/HT201359

"Some purchases aren’t charged to your Apple ID balance

If you're in a Family Sharing group, purchases are charged to your personal Apple ID balance. If you don't have enough Apple ID balance to pay for the purchase, the remainder is charged to the family organizer's primary payment method."

Family purchases and payments - Apple Support - https://support.apple.com/HT201079

"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. "

Family members can add balance to their accounts with iTunes/App/Music Store gift cards:

Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes - https://support.apple.com/HT201209

or:

Create and add money to an iTunes Pass - "At Apple Stores and select retailers, you can add store credit directly to the Apple ID that you use for App Store, iTunes Store, and iBooks Store purchases." - https://support.apple.com/HT203021