Hi C.T.S.
I am John J.D., a fellow Microsoft user like you.
It's important to ensure that both you and your son are signed in with your respective Microsoft accounts. To begin, kindly have your son follow these steps: Select the Start menu > Settings, then go to Accounts and choose Your account. If he's not currently signed in with a Microsoft account, he should opt to Sign in with a Microsoft account and enter his email address. If his account is already linked to an email address, please make a note of it.
Once you've confirmed that your son is signed in with his Microsoft account, try removing him from the family group, then proceed to add him back following these steps:
How to remove Family member: https://support.microsoft.com/en-us/account-bil...
How to add Family member: https://support.microsoft.com/en-us/account-bil...
If you continue to have trouble inviting your son to join your Microsoft Family after following these steps you can contact Microsoft Support. Here is the link: https://support.microsoft.com/en-us/home/contact
> Type a keyword and press Enter.
> Then click "Contact Support"
You can also try to contact Microsoft support using Get Help app.
> Press Windows + S key to open Search.
> Type "Get Help" and click Open.
> Type a keyword and press Enter.
> Then click "Contact Support".
Please let me know if you have any further questions or concerns.
Best regards,
John J.D.