storage

Please read the following support document -> What's the difference between device storage and iCloud storage? - Apple Support

If the issue is with internal storage, you will need delete data off his phone to gain back memory.

IF the issue is with icloud - then you can purchase more icloud storage to backup and sync to the cloud.

Links to refer to:

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support

What's the difference between device storage and iCloud storage? - Apple Support

Manage your photo and video storage - Apple Support

I sometimes really wonder why people keep asking this over and over again since there’s a difference between iCloud storage and the storage on your iPhone, I guess they don’t really know the difference. I sometimes feel sorry for them because they buy iCloud storage and then realize that they cannot use it and wasted money.

You rented extra iCloud storage and that does not increase the capacity on the iPhone.