OP
Besides getting a vet workup for ulcers, you may want to invest some time and patience.
This poor creature is a saint to keep on trying to do what is being demanded of him. He is telling you every way he knows how that he is in pain and the response is that he gets physically punished.
I know you bought this horse to ride, but can you just turn him out for a few weeks to just graze and be a horse and not a riding, jumping machine?
I would research and consult with an equine nutritionist and feed your horse a diet appropriate for ulcers , stomach and hind gut. It wont hurt him.
Iām not saying not to interact with your horse. Bring him in and groom him, pick his feet, etc.
Let him get to know you as his care taker and not just a rider who insists on pushing him beyond what he can endure.
I dont blame you for wanting to ride and pursue your goals; thatās why you bought this specific horse .
I donāt care for your trainer.
A real horse person would stop assuming that the horse is being ādifficultā and must be punished until the horse learns its lesson.
A real horse person finds the reason and if down time is required then the horse gets downtime.
Aside from ulcers which I believe is a real possibility given this horses previous history, you may want to get radiographs of his hooves.
Pain or discomfort in the hooves can definitely create an unwillingness to jump.
I hope my post doesnt come across as harsh.
You care about your horse otherwise you wouldnt have asked
Be the advocate for your horse. If you dont stand up for him, no one else will.
If you decide heās not the horse for you, then try to find him a non-jumping home.
Good luck to you and your horse. I hope you can find a solution soon .