Hi Mandiblabs!
This error happens because of how Apple handles payments within a Family Sharing group.
It could be that purchase sharing restrictions are on. If they are on, Apple routes all App Store and in-app purchases to the Family Organizer’s primary payment method.
Apple Cash vs. Apple Account Balance:
Apple Cash can be used for Apple Pay at store checkouts or in apps that support Apple Pay directly. However it cannot be tied to an Apple ID to pay for recurring App Store subscriptions or direct in-app purchases under Family Purchases.
To make this work, you need to fund his Apple Account Balance instead of his Apple Cash
First, send an Apple Gift Card to his email address or use your device to add funds directly to his apple account
Next, redeem the balance on your son’s device, you do this through the App Store
Finally, when he makes an in-app purchase it will charge money from his Apple Account Balance first.
Let me know how it goes!
-Owen