What has been done by the vet in the way of diagnostics? X-rays, ultrasound, MRIs? At this point, there could be all sorts of things going on inside this horse, that are causing this.
We had a TB colt that was just as nice as good be, other than was a cryptorchid. When he went to FL to be trained, he kept pulling up when he was being breezed, and after looking into it, they found the other testicle was under his spine, right where saddle and rider were, causing pain.
Could be a deep abscess, could be muscle tear, could be broken, or sprung ribs could be many things and not fair to horse to keep doing anything, until you have a definitive answer that there is nothing wrong.
At the same time, letting all of these others ride him, and get thrown, is opening you up to a major lawsuit if one of them gets killed, or God forbid, paralyzed.
Horses make me a better person.