Remove trusted device and keep on family sharing?

Are you and your son sharing an Apple ID? If so, create a new Apple ID for him.

Apple ID - create a new.      

Apple ID – Create         

Family Sharing.         

Family Sharing - Set up.        

When changing to the new ID, there may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos and/or in Photos/Preferences/iCloud. Then make sure full resolution photos are on the device/computer. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. See Winston Churchill's response in the below discussion. Also set up Family Sharing between the IDs to keep access to your apps/purchases. 

How to switch Apple IDs on your iPhone or iPad.      

How to transfer all files from one iCloud account to another.        

What to do after you change your Apple ID or password - Apple Support.     

You are welcome. You can remove the phone as a trusted device using the steps in the article below.

Apple ID device list -Check to see where you‘re signed in.    

If you don't want him to be able to change your Apple ID password, consider adding 2 Factor Authentication to your account. Also make sure his phone number isn't listed on your Apple ID.

Apple ID -Two-factor authentication          

 Two Factor Authentification availability.