The ones I've used and seen others used with success is Quietex. But that is just a band aid. There is nothing better than practice, practice, practice. If you need to pay for someone to bring a trailer out a couple of times a month (or more to work with him), you're going to be way better off.
My boy is deathly afraid of wind storms, which we have a lot here. I sometimes have to give him Quietex when all my other methods don't work (extra hay, extra love, music). So they can be used, but again, nothing can replace working with them.
Unless it weighs a ton... it's just a horse. Draft horse motto.