Hello PeteSett,
Regardless if you change him to a Parent or not, the Ask to Buy requests always go to the Family Organizer. The only way to change this is to kick him out of the family group, then he makes his own family group, then you move the child to his group, then you disband your family, and then you join his.
Approve what kids buy with Ask to Buy - Apple Support
"With Ask to Buy, when kids want to buy or download a new item, they send a request to the family organizer. The family organizer can use their own device to approve or decline the request. For example, if a child wants to buy an app, the family organizer can see the app and decide whether to allow it.
If the family organizer approves the request and completes the purchase, the item automatically downloads to the child's device. If the family organizer declines the request, no purchase or download will take place. If a child redownloads a purchase, downloads a shared purchase, installs an update, or uses a redemption code, the family organizer won’t receive a request."
Thanks.