Were you able to try Catty1's suggestion.

I should have responded to this thread sooner, sorry! because your cat may have what my cat had since the symptoms are very similar. Seshat fell (startled by another cat a the top of a climber), not very far, but landed awkwardly instead of her usual graceful way. She seemed fine, then suddenly she was on her side literally screaming with pain, writhing and stretching out her back, growling at me and screaming more as I picked her up and into a carrier to race her to the emergency vet. While there, she seemed back to normal. They x-rayed her, checked her out, and put a bandage on her lower leg saying she might have broken her toe. Back home, she jumped out of the carrier before I could stop her and then dropped to the ground with more loud cries and writhing. This went on intermittently all night and I took her to another vet the next day. The vet there said it sounded more like she had jarred her back and we could only wait for her to get well by herself. And, day by day, she did, although there were always times over the years when she would step between floor levels and I'd see her stretch out her back with some obvious pain.

All this to say, that if this is the problem with your cat, he is probably hiding most of the time but is able to move around when he needs to, so not as lost to you as you fear.