Hello, thank you for reaching out to Microsoft Q&A.
Ask your son to sign in with his account using the link below:
https://outlook.live.com/groups/
(Click Your Family > Members)
This should take him to the Groups page, where you can find the details of the family group members in Outlook.
Under “All members and guests,” select the organizer account and look for the email address credentials.
Try resetting the organizer account password, then sign in to the Family Safety account here to remove the child account from the family group. Or try recovering the organizer account using the account recovery form here:
Note: Only the person with the organizer role can add or remove members.
Let me know if you still need more help.