Apple has transferred my account to my son's address without reference to me. How is that possible?

Questions:

1) Do you and your son have separate and unique Apple Accounts?

Make sure that each family member has a unique Apple Account - Apple Support

2) Have you setup Family Sharing?

How to set up Family Sharing on your iPhone, iPad, or Mac - Apple Support

3) Was your Son’s Apple Account created from one of your devices by the process outlined here?

Create an Apple Account for your child - Apple Support

4) Did you setup Apple’s Purchase Sharing as described here?

How to share apps and purchases with Family Sharing - Apple Support