Most problems are caused by the owner. Horses are social herd animals. They take their cues from their companions. If one horse in the herd bolts, the horses usually bolt with them. They don't turn around and look to see if the wolf has really gotten that close. They just run.
Same thing with people. If you act nervous, the horse thinks he should be nervous too.
Take that flymask, open it up, plop it on his head, and close the velcro. Don't sit there and slowly open it, glancing fearfully at the horse every other second, and then slowly raise it to his head (extremely slow movements like that tend to make horses nervous. Maybe because it's so like the stalking movements of a cat...?), and soothe him when he tries to move away.
If you do it in a no nonsense kind of way, the horse probably won't even have time to react.
When you go to take the velcro off, just calmly and quickly open it. If he reacts, just send him in a circle while still taking the velcro off. It's really nothing to be afraid of.