Can I ask what the meltdown in hand consists of, ie bolting, rearing, striking?
I would take a very good look at his feet. I had a sweet OTTB who had been restarted and I was her second off-track owner, she was 5ish when I got her (I think, it’s been a bit). She was a stoic girl, nothing bothered her, I mean nothing, she was 5 going on 20. Then one day she bolted under saddle, no warning, like she had been hit with a cattle prod. She did this intermittently for a while, I mean HUGE bolts. I got off at a clinic and a pro got on, thinking I’m just blowing it all out of proportion and I’m stupid, and she bolted under the pro so hard she landed behind the saddle. I buted her for the second day of the clinic and everything was fine, so I figured it was pain related. I had my regular vet out, he did flexions and a general exam, palpated everywhere, nada, and the behavior didn’t present that day. Continued, had him out again, this time she bolted on the lunge, I mean WHAMMO tally ho, hold on for dear life, and then just as quickly, stopped. We started her on regumate and vet warned that the only time he has seen this behavior was a horse who was PTS and necropsy revealed some sort of massive tumor wrapped around his intestines that was causing who knows what sort of pain. Went to another vet who did sporthorses almost specifically and after describing everything, xrayed her neck (she cribbed) and back, freakin nada.
Then one day I came out and she was missing a whole side chunk of hoof wall from the nail holes down. She had massive WLD that had left painful hollows under her hoof walls. Resected most of her hoof wall and spent a LARGE amount of time treating and regrowing. She never did the behavior again and I wound up selling her to a friend as the family’s first horse and she stayed with them until she died.