I need to add a family member whose email address is no longer in use

You cannot have two accounts with the same email address. If you need to set up a family account, please refer to this link: https://support.xbox.com/en-US/xbox-one/security/add-family-member-on-xbox-one

Kindly use an adult (organizer) account to change your Privacy Settings but if you're not yet in a Family Group, follow the steps below to add your account as a MEMBER.

To add someone to your family group:

1. Go to family.microsoft.com.

2. Sign in with your Microsoft account (parent/adult account), then select Add a family member.

3. Select Member.

4. Enter an email address or phone number for the person you want to add, then select Send invite.

Note: If the person you're adding is already part of another family group, they'll first need to be removed from that family group, or they'll need a new Microsoft account for your family group.

5. Have the person you invited to accept your invitation from the email or text message they receive. Or, in the Notifications section, select Accept now next to Pending member. You'll be signed out so the person you're trying to add can sign in to their account and accept the invitation.

You will need to setup a new email account for the child

change the email address on his account to the new account

Setup your xbox live account with your email address

pull in his account under your xbox live account as a child account in under your master family account

I do this with several grand-children to help their parents out. This site should help out: https://support.xbox.com/en-US/xbox-one/security/add-family-member-on-xbox-one

Did you try this?:

1. Go to
http://live.xbox.com/FamilyCenter

2. Click Add Family Member

3. Enter the email address for the family member’s Microsoft account (formerly known as the Windows Live ID) and when asked, their password .

4. Add them to the family.

If you are playing on two consoles he will need his own account.

Create a child account and add family members on Xbox One.

• Press the Xbox button  on your controller to open the guide.
  
• Select Profile & system, and sign in to your account.
  
• Press the Xbox button  again to open the guide, and select Profile & system > Settings > Account.
  
• Select Family settings > Manage family members.
  
• Select Add to family.
  
• Select Add new.
  
• When the child is prompted to enter an email address, enter their existing email or, if they don’t have one, select Get a new email.
  
• Create a new @outlook.com email address that your child can use.
  
• Enter their first name, and then their last name.
  

The following link has more information about a child account:

Xbox Support Help Page - Child Account