MY 2 QH's would spook at darn near everything when we very first got them. After a few months... maybe 6 they had settled on their own quite a bit. Seem like once they were comfortable with us we then went on to "de-spook" mode. I used a step in electric fence post in the middle of their small pen and tied plastic bags and used hoola-hoops, I would toss empty milk jugs into the pen, I used some of my son's larger ride on plastic toys in there for a while... Just anything new. Now they're in great mental shape. I just didn't try to hide anything from them... I let the kids be loud maniacs around them (but outside the pen). I know not everyone will be able to do this but turning them loose in my yard helped a TON. We would mow and weed-eat with the horses out and they would take off about 20-30 feet but then get interested and investigate. It got to the point where we would have to shoo them off.
Time and Patience.
You can do it. Most importantly than any silly object is to prove yourself the leader. I love angel leaguers advice on the I stop game and I never ever ever give up grain without asking for the horses to behave and stand patiently first. I'm alpha and you'll look to me for help (and feed).
Eventually he'll look to you for guidance on spooking too... Give it some time.