lol this may sound kinda mean but when he spooks give him a good pop on the butt he'll think again before he does it next time. See some horses that spook for no reason see that they can get away with doing it its kinda like a bad habit you gotta break that bad habit some nice poppings are good. No its not cruel its just a way to teach him not to be scared.
We have a horse that was doing that and we did not "just pop him" we got the vet out to check him and it turned out that he has a cyst in his right eye. So I would suggest getting him checked.