iPhone Backup

I recently turned on family sharing for my parents as Dad’s iPhone had no space to back up. They joined and it all shows correctly. His phone shows his storage out of 2TB with plenty of space, and his next back up is projected to be 5GB in size.

Despite this his option to back up on his iPhone to iCloud is greyed out and the message below states there is not enough space to perform the back up. I’ve talked him through resetting back up, turning the phone off, resetting the phone, all of which have not worked. What baffles me is in storage his phone shows 2TB so it knows it has the space. Does anyone have any idea what’s going on?