iPhone storage completely full, don't want to loose photos, have icloud storage

Ok so I am trying to help my uncle who has an iphone 5s I believe. The iphone is completely full most of which is photos. He has subscribed to icloud storage and we made sure we switched on the feature to only keep the small photo sizes on the device. He has 200 gb of icloud available to him ( he just upgraded and it is much more space then he needs at the moment.) The issue is that if we try to delete his photos from his iphone it says that they will also be deleted from cloud storage. We want to keep his photos backed up somewhere of course.

The last time I dealt with an issue like this I could sync the phone to the mac and then select to delete the camera roll etc. I did not see any camera roll for photos and syncing with mac looked different to me as well as I did not see any options for deleting photos. I know apple is really pushing people to pay for the cloud storage but this is frustrating.

Is his iphone so full that it can't sync the photos he has to his icloud storage? How do we delete photos without deleting them from cloud storage like the alert message indicated would happen.