family sharing/apple ID's

First, remove him from family sharing. See Leave Family Sharing - Apple Support

Second, use How to create a new Apple ID https://support.apple.com/en-us/HT204316  and

How to switch Apple IDs on your iPhone or iPad

https://www.imore.com/how-change-apple-id-your-iphone-or-ipad

See the data implications below.

Some of these things may apply even though not changing an Apple ID: What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

 Keep in mind that some things available under a first Apple ID will not be available under a new Apple ID. For example, you won’t be able to transfer anything purchased from the App and iTunes Stores to the new ID. And, there are other issues to be addressed.

And see what Eric Root says on this matter here  https://discussions.apple.com/thread/251483565 and

what Winston Churchill says about it here: https://discussions.apple.com/thread/7452747

Third, invite him to join your family. See Set up Family Sharing – Apple Support

Are you and your son using the same Apple ID? If you are, then he doesn't really have a separate account. See if the steps in the article below will allow you to create a separate account for him.

Family Sharing and Apple ID for your child

If necessary, contact Apple ID Support for assistance.  See the Apple Support articles below for contact information.  

Apple ID - Official Apple Support.    

Contact Apple for help with Apple ID account security - Apple Support.   (Link doesn’t always load)  

Get Support          Work through the screens to get a chat option. You may have to pick a slightly different problem.

Starting an online chat with Apple Support

Apple Support schedule phone call

https://twitter.com/AppleSupport.          People have posted it may take an hour or more to get a response.

You are very welcome. Nothing about what you want to do is easy. And, without getting into detail, I don't think the first thing you mention will work. Keep in mind that you can't remove a child age 12 or younger without transferring them to another family. On the second, I'd rather not comment because of the subterfuge involved. I understand that you motivations are not nefarious though.

Ok, I was hoping that wasn't true. That complicates things significantly. Let's see what others have to say.