Have you checked him for ulcers? I am thinking ulcers may be part of your problem or be the problem. The fix for ulcers is easy and not that expensive. It sounds like he might be reacting to the pressure you are putting on him. That might be getting his stomach to bother him. Most TB’s love to please their person and enjoy work. Have your horse scoped is simple and can be very revealing. Do him the favor of consulting a competent vet before you give up on him.