Rhythmncruise nailed what I’ve thought throughout this thread. You have done all due diligence as regards your horse’s health, except for a bone scan and brain MRI, if possible.
Having a mare who, at age 5 was dangerous enough that I considered euthing, I contacted, as a last resort, a local woman who was an acolyte of Buck Brannaman. Mind you, if that didn’t work out, the mare would go.
In her particular case, she was beligerent, impossible to lead, at times mindless, though I could ride her with little problem, except some silliness.
Trainer and I worked together on her. Mare was agressive to other horses in turnout and we determined early on that Boo needed to be put in her place by a horse before any reasonable progress could be made. The contretemps with a dominant appy mare lasted 15 seconds of squealing and hard double barrel kicking, after which Boo backed down.
Only after this could her training begin. She was pushy, smart, testing all the time, striking out, kicking out when under pressure and reactive.
Once we had her mind using consistent,firm and progressing challenges on the ground, she became a sweet, happy and useful mare.
She was dealing with pain issues (locking stifles), which made her proctective, but she had to learn to not take her fear of pain out on us.
These are not your horse’s issues, particularly, but I just wanted to emphasize that trust was paramount in her development. It would seem that your horse has learned, somewhere along the way, that trust is not something he has, inspite of your excellent intentions.
Trust=respect, here.
Something I would have high on my list to try to help him would be to pull his shoes and turn him out with a couple of others. They will play. He will get nicks and bites, perhaps kicks. He will run, pee, just be a horse. He doesn’t need shoes in winter, he can deal, most horses can.
A last question…has he shown any of this behaviour when not being led? As if, perhaps, when turned out, he gets a twinge of pain? Have you round penned him?
I hope things get ironed out for you both and please keep us posted on his progress. There has been lots of good advice for you.