I just read a post by someone who said "most horses don't like fly spray and there is nothing you can do about it". That is soooo not true.
Yes, you can fix thi and its easy. Fill a spray bottle with water (so you don't waste your expensive fly spray. Put your horse in a rope halter (not nylon) and lead (don't tie him). Begin squirting him on his front legs (from the side). If he moves, fusses, backs up, keep squirting and move with him. The very second he stops moving, you stop squirting. Do this all over, when he relaxes, you immediate stop squirting. Eventually, he'll stop moving when you fly spray him.