When I was training foxhunters, we had an older horse who went under very quickly.
I rode him to exercise him one day and his hind end kept slipping. So we had the vet out. The vet noticed a scrape on the inside of his hind leg and blamed it on that. I was skeptical. I've never seen a scrape cause the hind end to fall.
The next day he was worse, to the point that he couldn't get his hind end up off the ground. We finally pulled him up and put him in a stall. I wrapped his hind legs because he kept scraping them while trying to keep his balance (scariest moments of my life, being under a horse to wrap legs who couldn't keep his balance!)
Next day, he was worse. I put him on the trailer and took him to the vets. He made it all the way there and then fell while coming off the trailer. The vet put $500 into him (which I thought was cheap) without knowing what was wrong, and I called the boss. The boss said to bring him home so we could shoot him.
I wasn't going to put the horse through another trailer ride, so I told the vet to put him down right there.
We never knew what went wrong. EPM or wobblers were two ideas thrown around, but the boss didn't wait for a diagnosis. The second he knew he was at the vets, he told me to just shoot him.
Wollbers usually effects young horses, but it can effect older horses, especially if they have arthritis in their spine.
I hope your horse comes back healthy. At least you are giving him a chance, something my boss never gave this one horse. If his bones grew wrong and his stride is wonky to begin with, it may just be setting the vet off on the wrong track.