There are some audio CD’s, with a variety of noises that typically upset horses. The idea is you start with the volume low, then increase it as the horse becomes acclimatized to that sound. If you start when the horse is eating, he has something he likes to distract him. There are CD’s typical show noises, such as loud speakers, CD’ with guns going off, which are used to desensitize police horses, and a variety of others. One of my long term friends rides her Arabians in parades, and one year she took the horses out west for some type of a circus parade, complete with lions & tigers (oh my!), and her Arabians were unfazed. Arabians are well known for being only to well aware of what is going on in their immediate area, so I suspect these might work for your horse.
If you are interested, I can ask my friend where she got her CD’s.