If this horse hasnt been handled much, or at all, only one thing is going to help you and that is time.
I got my horse when he was 10months old, when I went to look at him he was great, haltered, trailed, respected your space, stood still for grooming, picked up feet, ect......well when I got him home it was like a switch went off. I couldnt get him to do a thing, nothing!!
I started going out to the barn everyday and literally just sat next to him and talked to him for a few weeks. After I gained his trust, and eventually respect, our possibilities are endless and Im able to do just about anything and everything with him now. Now a year+ later, I was able to get on him without any issues....good luck!