My son is listed on my icloud sharing account as a member but is not able to access storage. It says NOT USING when you click family members listed in the account.

Under the settings on my phone (main phone holder the account is in) - it lists under iCloud+ manage this subscription - My husband, myself, and my daughter. My son is listed but under his name it says Not Using. I tried sending another invite and it says already a member. But he is using his own storage plan, he needs to be a part of ours as a family. Help!