This is just my experience with Hoover, but it may help in some way:
Hoover was first an Amish horse, and they beat him all over. His legs, his head. After that, he was owned by a nice man who kept him as a pet, and pretty much let him do what ever he wanted. When I bought him, he was confused. He didn't know his boundaries, or his "place" in his human "herd." He never knew if he was about to be beaten, or be allowed to be alpha and push everyone around.
After about six months of having him, it all came to a head. I was brushing him in stall, and he pinned me in the back corner, came at my head snapping and dancing. He scared me so bad I socked him in the nose (only time I have ever used a fist on him). He rocked back in surprise, then came back at me. I managed to get the door open and dive out into hall, but he had bitten my arm purple. My BO and trainer went in and gave him a piece of his mind, and Hoover kicked at him.
I put him in training with my BO, and the turn around was amazing. After a month, he calmed down. He is happy and content, because he knows his position and what is expected of him. He had been confused, and that made him scared, which made him act out.
So hopefully, your experience is similar to mine and the trainer will do wonders with him. Hoove was trained by a form of natural horsemanship, in which we teach the horse we can make them move, and then that it's lets work to listen to us. We never use more than a cotton lead rope in training generally, or hands and voice. Hoover is now so mortified if I raise my voice to him he hangs his head and refuses to meet my eyes for a moment, with a look that is so apologetic like "I'm sorry mommy, I do know better...I love you..." Heh. Then I hug him and tell him I still love him, I just don't like it when he acts that way.
"Sit tall in the saddle, hold your head up high. Keep your eyes fixed where the trail meets the sky. And live like you ain't afraid to die...don't be scared, jut enjoy the ride." - Chris LeDoux