Unable to disband family/add child to new one

We've recently added an Apple One family subscription to our account (for four members). Three out of four of us were able to accept the invitation and join the family account that held the subscription. My son (under 13), however, is unable to accept the invitation - he gets a notice on his phone that he's already in another family. That family account is linked to my iPad. I've been reading that I should be able to go into the iPad settings > choose Family > and select "Stop Using Family Sharing", but that button is greyed out. All of the instructions that I'm finding online indicate that in order to disband the family account, he has to be invited to another one, but this prompts the message that he's already in a family. What am I missing here?