I am really sorry to hear that, I really can't offer any help, but I am really sorry and hope they are able to figure out what it is.
My Aunt had a high level Dressage horse that was having issues similar to it. They don't know what she did to herself, my aunt had a vet out and chiropractor and everything, and nobody could figure it out. She was 100% sound according to the vet, but her back was all out of whack.
She ended up not being able to fix whatever it was, and retired her to a broodmare.
Now, after having a baby and resting for several years, she is fine to ride.
Who knows what it was!
I hope they're able to find the problem with your poor guy!