Can you turn off purchase sharing for a member of the group and leave it on for everyone else

From that support document:

Which payment method is charged for purchases that family members make?

When Purchase Sharing is turned on, everyone's purchases bill to the family organizer's payment method.* If there's an issue with the payment method, the family organizer can change or update their payment method.

If you're the family organizer and you don't want to share a payment method with your family, turn off purchase sharing. When purchase sharing is turned off, you can continue to share subscriptions like Apple TV+ and Apple Arcade.

* Learn how purchases are billed if a family member has Apple ID balance.

The organizer undertakes to pay for everything not covered by store credit, unless they turn off purchase sharing altogether I guess. Other family members cannot add credit/debit cards and are limited to store credit.

tt2

See How to share purchases with your family - Apple Support. I think each family member has control over whether or not their own purchases are shared.

tt2

You cannot transfer store credit from one person to another. You could purchase iTunes Gift cards or vouchers to give to your son to cover the costs of the services you wish to pay for, or pay him back, or ahead of time, in some other fashion. E.g. bank transfer, standing order, etc.

tt2