Remove invalid child account from family sharing

Hi,

I'm trying to remove an invalid, old child account from Family Sharing. My son lost his phone some time ago and since then has got a new phone with a new phone number. I added the new account to Family Sharing with a new email address.

I want to remove the old invalid account but cannot despite following Apples Help pages.

In order to transfer the defunct account to a different Family he needs to accept the invitation sent to the old email account. When clicking on the link to accept, he gets a message that he is already in a family and cannot. As the Family organiser I do not get any messages of the request nor the ability to accept/approve the invitation.

We cannot log into the old Apple account as he does not know the old password. Reset password does not work because:

1. he no longer has the phone number registered for the old account;
2. he does not remember the password.

It's a chicken or egg scenario. To transfer the account I need to log into the account. I cannot log into the account because I need to reset the password. I cannot reset the password because I no longer have access to the account.

How can I proceed?