Seek a trainer and start from scratch. All this behavior is communication to you something is wrong. He is either in pain from ill fitting tack, something hurts he is telling you loud an d clear, or he has a serious behavior issue. If your having the vet come out run the lymes blood test as a precautionary it’s not expensive. If it’s ulcers you might need to treat with malox twice daily for a couple weeks to see if he seems more at ease. Thoroughbreds can be hard keepers but and most people don’t hear this enough. Horses don’t need grain. I would take him slowly off the grain and introduce a hay stretcher pellet. Then increase the amount of hay. he should be getting enough to keeps him busy and keep the weight on. If you rule out everything medical I would start with ground work with fear. buy a tarp they are CHEAP! He needs to accept it walk on it, wear it, be round penned daily to keep his energy at a reasonable level. You can’t give a kid candy and then be shocked when he doesn’t want to sit and learn his ABC’s. Become familiar with his personality on the ground and when he softens to a place where he is a good listener then ride him. I would also re-start with jointed loose ring snaffle the midest of bits. Train him from the ground to give to the bit. I don’t like hackamores they can be restrictive. You should never have to up bit. Instead train him to accept the mildest and be soft.