How to change the name on the iPad devices.

That wasn’t a good idea at all. Now your purchase are linked with his Apple ID. If you create your own Apple ID and erase the device now, you will have to start all over again and purchase everything that you may have already paid for all over again. The only alternative to that is to start Family Sharing so that you can keep the purchases or download them again using Family Sharing.

This explains how to create an Apple ID

How to create a new Apple ID - Apple Support

Some information about Family Sharing

Family Sharing - Apple

If you want to use Family Sharing, you either have to erase the device, start over and create your own Apple ID and then set up Family Sharing, or if you don’t erase the device, sign out of every Apple service on the iPad, then create a new Apple ID, sign in everywhere, and then set up Family Sharing.

These are all of the services that you should sign out of on the iPad. Not all will apply to you.

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

You need to turn off Find My Phone if it has been turned on. You must sign out your son. Then restore the device as new, after which you can configure it for yourself.

Essentially, you will be restoring the device to factory state: Selling/Giving Away Device or What to do before selling or giving away your iPhone, iPad, or iPod touch.