How do I delete my son's Apple ID?

Is your son a child according to Apple?

One way to do this would be to log into the unwanted "child" account and request deletion of the Apple ID. You should have the ID and password from when you first set up the account.

Another way would be for the child to make the request.

From: How to delete your Apple ID account - How to delete your Apple ID account - Apple Support

Section: Can I delete my child’s account?

"Your child can also request deletion of their Apple ID, which requires your approval.

The approval request will be sent via email to the primary Apple ID email address of your family organizer account. When approving the request from this email, you will go through the process of deleting the child account."

If the child's account password is unknown, see the section in this document about using a family Organizer's device to reset the password --> Change your Apple ID password - Apple Support

If you have problems doing that then contact Apple about this request: Click here --> Choose your country or region - Official Apple Support Select your country, then a product and work your way down the menu to the desired issue. If you don't see one then try a different product until you reach a telephone or chat help request and ask to be redirected.

How to permanently delete your Apple ID

Sign in with your Apple ID at privacy.apple.com on a Mac, PC, iPhone, or iPad, and select “Request to delete your account”.

When you request account deletion, you'll receive a unique alphanumeric access code. Make sure that you keep this access code in a safe place. For a short period of time while your request is being processed, you can contact Apple Support, provide your access code, and we’ll help you cancel your request so you can resume using your Apple ID.

How to delete your Apple ID account - Apple Support