I never post here but I don't think this problem is discipline related. And I don't think the OP is doing anything wrong. Even the trainer, who has had obvious success with overcoming his past and getting him engaged and happy with his job, can't flesh out the change in behavior at home. I agree the problem is psychological. But I think it's in the horse's head and not the owner's.
Thirteen owners in 20 years... is a flag. But... he's finally found some stability with his current owner and a trainer that made a connection. And after six years at the same home he should not be having environmental problems. Yet he's a different horse in different environments, even though his "job" is essentially the same. So what jumps out at me as the missing ingredient between the two? A sense of competition.
Thoroughbreds stand head and shoulders above other breeds for their competitive nature. And it sounds like this horse is one of the few who really thrive on it. The missing element is the presence of other horses and the anticipation of competition.
This horse appears to be a natural specialist. I doubt he can happily do double duty as a performance horse and a pleasure horse. For him it would be one or the other. But not both. And practicing a job he likes so well under two so disparate environments (home and away) can tend to pull down his overall performance if he's "made" to when he has no sense of urgency. That leaves constant trailering to a busy facility or boarding at a training barn to provide the additional consistency he seems to need. Both could be possible deal-busting additional expenses in time and money.
Granted. Most any horse should be consistent no matter when or where he's asked to respond to his learned cues. But I don't think this one is just "any" horse. It's possible if he had found the right owner and trainer 14 years ago history might know his name as a top level competitor. But he's finally come to the right people for him to be with a bit late in life.
Leaving him at the facility for 30 days where he does his best work to judge his consistency would be a telling experiment. I think you'll find it's not the condition of the ground, pain in his legs, a hesitant rider or the view across the fence at home that makes him such a different horse. He's got nobody to show off in front of at home. And he's one of those rare horses that needs to have that drive to be fully engaged in his job.
(ETA- Another thing to try as a "control" for the experiment would be to find another farm similar to your own, with just one other horse in residence and a practice arena to take him for a riding session. See if his responses are similar to those at home or if he composes and focuses himself to the level you get when at the trainer's place. If he remains the same hollow and disinterested horse there you'll have a better idea about his mindset. If he DOES perform much better at a place similar to your own, then I would start looking at surface conditions in his exercise area.)
Personally, I think a decision is at hand. Either keep him at home and live out life as a "hollow" and somewhat disinterested pleasure horse or move him to a training barn where he can get all the motivation he desires to be what you really want him to be. I don't think half measures will change anything with this guy. I think you have one that may be really special. ~FH