Start by cataloging all the symptoms, both under saddle and on the ground. If you have a timeline and videos from a few months ago, years ago that might also be useful.
You are getting all kinds of advice on what the problem might be, but once you get to an expert, your time (and theirs!) is best used by defining the problems you have seen, not what or where you think the problem might be.
You might also ask the two trainers who worked with him to describe any issues that they had. Chances are there was more than what you heard, it’s just those were things they may have originally thought would be resolved with more training, but they may give the vets more context today.
As a side note, if a horse flunks out of two good trainers, there is a chance they encountered the one type of horse we all fear: the one that does not care if it kills itself while trying to kill you. There’s a lot of horses that can really hurt you if they have a behavioral issue or a medical issue, in fact pretty much all of them fall into that a category. Most try very hard not to hurt people, some don’t care about you but still have a healthy sense of self preservation. A very small fraction do not. I’m not saying this horse is it, but it’s something to be aware of, and possibly something to question those trainers about.
The fact that he was a lesson/trail horse certainly doesn’t sound like like that type of horse, but it may be that pressure is the trigger. If he feels like he is in charge, it’s all good, he’s a player, and maybe that was just the circumstances of his old job. Nothing they did pushed his buttons, and maybe they did get a sense of what he could be like and moved him along. It could have been not terribly ethical or something they genuinely thought would be easily fixable with an experienced rider. But now pressure is applied, and there is a reaction. Pressure in this sense is a training tool, it could be physical pressure, such as stand still for a minute longer than you want, yield to the bit, yield to the leg, followed by a more insistent aid if there was not any acceptable response. Somewhere along this training spectrum is usually where you discover this type of horse, and once you’ve found him it’s not easy to unfind him so to speak. He’s figured out what works and he doesn’t care if he dies for his cause. I’ve dealt with one of those in my life and she was beyond terrifying. A 2 year old in training, she came to my care after she chose to run straight into a wall in the shedrow (hurting her rider) then she ran out to the track and took up a section of the old wood rail with her shoulder (300 stitches and she was still sound). After she recovered only the farm manager would work her. When she went to the track, after a couple gallops with the trainer’s normal riders, he decided that only he would ride her because he sensed the same thing we did. She made it about 1 month in before she damn near killed him, he ended up in the hospital with ribs and punctured lung and multiple other injuries. She broke to the infield of the training track once again, taking up the rail with her shoulder and continued on to the infield pond where she drowned, which was probably best for all concerned. At the time I was young and couldn’t put a name to it, but there was something about her that had all the best horseman on their toes. Over the next few years her half and full siblings came through the training barn and every last one of them was foot perfect and nice race horses to boot. I even found one of them in the tail line of a derby runner decades later.
