Put him on a long lead, maybe 20 feet. Take a training staff or longeing whip and tie a plastic bag to the tip, like a plastic sac from Walmart. Start moving it around near him and see how he reacts. Once he starts reacting, keep moving it around, but don't get it closer to him. Once he decides it won't hurt him and he starts to relax (he'll probably lower his head a little and lick his lips and stop moving his feet), the pull the bag away from him. This rewards him for relaxing. He begins to learn that the way to get bad things to leave him alone is to relax. Progress little by little until you can run that bag on the staff all over his body and he remains completely relaxed, even bored. Up around his withers, over his head and face, etc. Like I said, progress little by little. Do it in stages. Once he relaxes with the bag touching him in one place, pull it away and remove the stimulus. Do each stage until he doesn't react at all. After that, start doing it with other things. Eventually he'll begin to react less strongly to things that scare him. Some horses have a stronger flight response than others, but you can minimize it and make him a much safer horse like this.
When I'm in the saddle and my horse spooks at something, I speak to her in a gentle tone, sort of like speaking to a child, and urge her toward the "monster". I say, "What is that?" and repeat it over and over, while urging her forward with my heels and keeping her head pointed at it with the reins. Once she gets close to the "monster", I let her sniff it and look at it until she relaxes. She has developed a trust in me, to the degree that now when she spooks at something she tends to automatically start making her way carefully toward it with very little urging from me. Ears pricked and cautious, but moving toward it. Her fear has become more curiosity than fear now. She has a strong flight response, though, and I still get a surprise once in a while, although she no longer runs. She just jumps a little.