Hello,
It appears that the Google Pay issue that you are encountering is not related to Google Cloud Platform. This forum is meant for general discussion of the platform, not specific technical issues or any anything which is not related to Google Cloud Platform.
For any issues related to Google Pay you can visit this Google Play Support page where you will find relevant support information.
You can also use family payment method on Google Play where by turning on purchase approvals you can approve or deny purchase requests of other family members as a family manager. It appears from your initial post you are already aware of family payment method.
As a temporary measure you can contact Google Play Customer Service number which is mentioned here at the Online Assistance and Customer Care section to remove your credit card or payment information from your son’s Google Play account. You can also request to remove any pending transactions if there are any which is subject to approval by the Google Play Support team. Please keep your son beside you while making a phone call as the account belongs to your son.