I agree with everything Loosie said. The approach and retreat method has worked great for me - reward the slightest flicker of interest or acceptance at first. Have you tried Linda Tellington's TTouch methods? There are some simple touches that horses love that help them relax. You could use the approach and retreat method, start with TTouches on his neck and shoulders and work rearward and up, as he learns to enjoy the touches, so that eventually you will be working with his back.