If you are a Family Sharing organizer then any purchases he makes could potentially roll over to the payment method you have entered for your account. For how payment works, refer to How to share purchases with your family - Apple Support
As for the Ask To Buy issue, if Ask to Buy isn't working, read the suggestions in the document: If Ask to Buy isn't working - Apple Support
If a problem persists, particularly over several days, try contacting Apple for assistance:
Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
"See a list of Apple phone numbers around the world."
Click here --> Contact Apple for support and service - Apple Support