Injections into joints help to maintain the synovial fluid within the joint capsule. Synovial fluid is responsible for lubrication of the joint and maintenance of articular cartilage, or the joint surface. If you’re giving injections, and see no improvement, my guess is that it’s not within the joint itself, but related to the musculature surrounding the joint – tendons etc. I wouldn’t be surprised if his hocks are sore now too because his stifles were sore – inappropriate ambulation causes harm to other areas. Bruises and stresses to muscles in horses can take three weeks to heal. What it sounds like is that at your trainer’s, your horse may have been overworked a bit at the canter (perhaps his conformation makes a slow, collected canter hard, or he already had some underlying soreness that was exacerbated?) and the daily work compounded into a bigger problem, that is now plaguing his entire back end.
In short, I’m guessing he was really sore – and to continue working, he compensated which caused hock soreness (and probably back soreness, which would further contribute to stifle problems, eek.) Definitely rest him. Give him six weeks of ‘me’ time and spoil him a bit, do fun things in the arena that are strenuous, so that he stops thinking, “Work hurts!”. (My OTTB is partial to orange cones, thinks they’re fascinating, as are soccer balls, she kicks them.) Massages will help too. That same OTTB fell in our round pen, at the canter, and had a deep tissue shoulder bruise. It took four weeks before she wasn’t short strided in the front, and that was after a week of bute, cold water therapy, and rest. I hope this helps a bit, I understand the frustration. But I think we sometimes ‘over do it’ with our horses, trying to give them the best possible care when all we’re doing is meddling. I know, personally, I do this all the time. I sincerely hope your boy gets better, and sorry to hear about your loss in October. Best of luck to you!