Hello AndroidMom,
It looks like you joined the Apple Support Communities from a great link: Approve what kids buy with Ask to Buy - Apple Support.
As it shows there, Ask to Buy was set up by the family organizer of the Family Sharing group that your son is a part of. To disable Ask to Buy, you'll want to reach out to the family organizer of the Family Sharing group and ask that they remove that for your son.
Who can use Ask to Buy
Families can use Ask to Buy after they set up Family Sharing. The family organizer can turn on Ask to Buy for any family member who isn't an adult. It's on by default for any children under 13. You'll be asked to set up Ask to Buy when you invite anyone under 18 to your family group. (Age varies by country or region.)
If a family member turns 18 and the family organizer turns off Ask to Buy, the family organizer can't turn it on again.
When you speak with the family organizer, you can as them to follow these steps to turn off Ask to Buy:
How to turn on or turn off Ask to Buy
If you're a parent or guardian, use your own device to turn on or turn off Ask to Buy.
On your iPhone, iPad, or iPod touch
1. Open the Settings app.
2. Tap your name.
3. Tap Family Sharing.
4. Tap Ask to Buy.

5. Tap your family member's name.
6. Use the toggle to turn on or turn off Ask to Buy.
On your Mac
1. Choose Apple menu > System Preferences, then click Family Sharing.
2. Click Ask to Buy in the sidebar.
3. Select or deselect the checkbox next to the child's name.
On your Mac with macOS Mojave or earlier
1. Choose Apple menu > System Preferences.
2. Then click iCloud.
3. Click Manage Family and select your family member's name.
4. Select Ask to Buy.
Kind regards!