It's telling you that there is a Family Sharing account that is already set up that is using your Apple ID. Are you certain your son has not signed in your Apple ID and set up a Family Sharing account with it?
Can you provide the exact text of the message you are getting? Just put in XXX for the Apple ID information, but so much of the rest of the text is blocked out, I can't tell exactly what the message is saying.
Cheers,
GB