Q: Music purchased not coming off son's loaded iTunes gift card, instead off my family share credit card.

This

Family purchases and payments - Apple Support

includes

Making purchases

After you set up your family, any time a family member initiates a new purchase it will be billed directly to your account, unless that family member has gift or store credit. First, their store credit will be used to pay the partial or total bill. The remainder will bill to the family organizer's card. As the family organizer, any receipts generated by the transaction will be sent to you. Learn more about how iTunes Store purchases are billed.

If you are doing that and the give balance is not being deducted then contact iTunes:

Report a problem with an item purchased from the iTunes Store, App Store, Mac App Store, or iBooks Store - Apple Support