It sounds like it is a behavioral problem. I would suggest starting again with the basics and rebuilding his confidence with the saddling and mounting process. When you begin to saddle your horse and he starts to become stressed, stop what you are doing and reassure him. Don't remove the saddle. Once he calms down, continue the saddling. Remember to take small steps, which may take a while. Continue with this process until he is comfortable with saddling. This may take several days. Make sure you complete the saddling process each time you start. Over time he will become less stressed. He should become quieter each time you saddle him. The same process needs to be followed for the mounting process. Take it quiet and slow and reassure him every step of the way. As you step up in the stirrup, stand in the stirrup until he quiets down. Once he is quiet, sit down in the saddle and let him settle in. I would dismount and repeat the process, praising him when he quiets down and accepts the process. Be very methodical during this process, going slowly and quietly.