Son's Apple account not using iCloud+ Family Sharing

My 12yr-old son has an Apple account set up on his Microsoft Surface laptop. He is part of a Family Sharing group with me and my wife. He used to be able to share our iCloud+ subscription, but that mysteriously stopped. We've tried a bunch of things, but nothing seems to work.

On my devices, it shows that he is part of the iCloud+ Family Sharing group, but next to his name it says 'not using' instead of a GB value. There are no outstanding invitations.

On his laptop, we have tried to verify his account, but it won't send the verification code to my phone. We've tried verifying through Apple Music on his laptop, and that's worked, but it hasn't made difference to the iCloud+ issue.

Any help appreciated.