Review Apple ID Phone Number

My son (a minor) has an iPhone and we are connected through family sharing. I have many controls over his settings. He is currently in another state attending school and he notified me that he received this notice on his phone:

Review Apple ID Phone Number

Confirm that +1 (•••) •••-••46 is still your number so you can always access your account.

First question: What is this?

Second question: That is my number - not his number. Why would the notification come up on his phone with my number and not his?

He has an iPhone 8+ running iOS 14.0.1

I have an iPhone 11 Pro Max running iOS 14.1

Thanks for any assistance.

Heidi