Horses are prey animals. It is common (when first learning to stand for a bath) for them to get antzy when "rain from the water hose" starts hitting them in places they feel vulnerable.
Some horses take longer than others to get used to showers and it will just take a lot of patience and using a showerhead nozzle on "soft spray" for awhile.
Some horses have tickle spots and it's a knee-jerk reaction when the water hose hits them in those spots. Typically the flank area, under the belly, between the hind legs.
Some horses have injuries and they can't stand to have the water hit them hard where those injuries are. I have one like that.
I am all for buying a cheap showerhead nozzle at WalMart and getting him used to a soft spray first.
Also, I never-ever tie my horses for a bath. Never have and I don't have a wash rack. I bring them out into the grass and drop the rope so they have the freedom to swing their head to get flies or take a step forward or backward if need be. I have two senior arthritic horses and they need to be able to shift their weight and move.
So my horses not only had to learn how to get a bath early on, they also had to learn that "head UP!" means just that. "--stand in the middle of that candy patch for your bath and keep your head up; you're a long way from starvation" - lol
These days, I send my 23 yo out the barn door with just a halter that isn't even latched. He goes to the spot where he wants to get bathed and he waits for me to get my stuff and get started.
But that all takes work, time, plenty of patience and give/take relationship with the horse. Even though all my horses have been taught to stand in cross-ties, it was only for discipline.
I would not want my head in a set of cross ties and I won't put my horses in them to do anything. They have all learned, over time, to just stand still, even if I am underneath them trimming their hooves - they don't get tied for that either - they stand with just a halter for the whole event. Regarding fly spray.
Certain fly sprays can burn the horse, even though there may not be any physical evidence.
You could try putting water in an empty (and clean) spray bottle and get the horse used to the spray effect with water.
Once that is done, and the horse wants to move away when you use the fly spray, it may be the fly spray and I would try a different brand.
It isn't the main ingredients in the fly spray bottle that cause the problems; it's those "other ingredients" and the amounts of each.
I have one horse that always tolerated Bronco until this year and he welted up the first time I used it. He now gets sprayed with Zonk all the time.
My Arab will move sideways away from me if the fly spray is stinging him. He also gets sprayed with Zonk.
I could probably spray the other two with diesel fuel and it wouldn't bother them - lol
Good luck, be patient - maybe more patience than normal, depending on this horse as compared to your other horse