iCloud+ family usage shows incorrectly

"iCloud+ family usage shows incorrectly: I am family coordinator and we have a 50gb iCloud+ account. All iPhones apart from my son’s show a total family iCloud+ usage of 31.9gb of 50gb. However, my son’s phone shows family iCloud+ usage 47.5gb of 50gb and so he can’t do backups. How can we get my son’s IPhone 15 to show correct usage? He is on the latest iOS"

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Remove, then Add Him Back:

Removing from Apple iCloud+:

1. Go here: Leave Family Sharing - Apple Support
2. Scroll to: Remove Someone from your Family Group
3. As it Reads: Learn the different ways you can leave or be removed from a family group and what happens when you stop using Family Sharing.

Adding Him Back:

1. Go Here: Add a Member to a Family Sharing group on iPhone - Apple Support
2. As it Reads: The family organizer can easily invite family members who have an Apple Account or create an Apple Account for a child who doesn’t have one yet.