Best I can think of is to ask your vet if there is something outside the normal stuff you can use for him. Is he in during the day? If so, does he have a fan? Fans help. I also spray the entire stall when I bring mine in – anything to cut down on the number of flies! We have a couple of horses who are so sensitive that they will rub off all their hair if let outside, so they are in all summer. A few of the others are in during the day, and out at night. If he has a run-in, can you hang screen flaps across the front? They are expensive to buy, but cheap to make, and all you need is a measuring implement, scissors and magnets (or some other way to attach them at the top). Instead of a fly mask (mine won’t wear them either), what about trying one of those ear net things, and leave his halter over it? My idiot won’t wear the regular fly mask, but seems to like the ear net things (thinks they are really cool hats) and the Cashel Quiet Ride mask (easier for him to see through). If you can get rid of the sores, perhaps you should use either the Swat or the one Absorbine makes (don’t recall name) on the insides – not deep, but just the area easy to see (another ask you vet thing – don’t rely on me).
It is never the horse's fault