Just stay consistent. You can try the sponge idea, otherwise as you are hosing his right side and have intent to hose his neck, the moment he reacts in fear, remove the hose and hose off somewhere he doesn't mind. Then repeat. The more times you add the pressure of the hose and then take it away when he gets frightened the more times he will realize it isn't going to attack him. That when he gets afraid, the pressure will go away so in time he will learn the water on his neck isn't a scary thing.
Pay special attention to the area where he starts to react. It probably happens way before you reach his neck, which is why when you get to his neck his reaction is loud. So watch him as your hosing him, watch for the 1st spot where he tells you what you are doing is not okay with him. Could be on his shouulder, could be farther back. But watch for that place, he'll get it. Just don't push it on him and he will learn to tolerate it. :)
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.