Hi there and thanks for your question!
That is certainly an odd behaviour!
How long has your cat been doing this for?
How often does he do it?
This is very puzzling! I too have never seen anything quite like this, but I would love to do some research for you.
This may take me a few minutes, but I should have some type of answer to you within the next 30 minutes or so...
OK...first I'd like to thank you for teaching me a few things! I have just done some research for you on Veterinary Information Network where vets talk about interesting cases and I have a few thoughts for you.
The most common cause for this is something called lumbosacral syndrome. Now usually we can see evidence of lumbosacral syndrome on xrays, but not always. However this condition usually improves with pain medication, which doesn't seem to be working for your cat.
The next thing that can cause these symptoms is problems with the anal glands. Did your vet check them? If not, it is a good place to start.
I'm assuming he is on flea prevention? If not, it would be worthwhile to try some. I have seen some cats who are so allergic to fleas that even one flea bite can send them into a frenzy.
And finally, you can ask your vet about something called feline hyperesthesia syndrome, which you can read about here:
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would highly advise contacting your regular veterinarian.
Hmmm...that I don't know.
The only other thing I can think of is that this is a bizarre behavioural problem. Has your vet talked about trying a behavioural medicine at all such as amitriptylline or clomicalm?
You may also want to ask about a referral to a neurologist or a behavioural specialist. Both of these specialists will have more experience with unusual cases like this one.