Hey DebDon2015,
You found the best Apple Support article to start troubleshooting if Ask to Buy isn't working. There were a few tips in the article that may be relevant to your issue since you gave your son your previous iPhone. We recommend checking these two steps to see if they help.
Try changing the name of your device, and check that you and your child's devices have different names. To see and change the name of a device, go to Settings > General > About > Name.
Then, double-check this:
Make sure that you're signed in to the App Store, iTunes Store, Apple TV app, Apple Music app, or Apple Books using the same Apple ID that you selected for sharing content with your family.
Finally, it sounds like everything is working on his end in submitting the request. Try focusing on your iPhone for a resolution. After you have confirmed your iPhone is updated, Family Sharing is set up correctly, and the above settings, try restarting your iPhone. A restart can resolve many minor software-related issues.
Kindest Regards.