Nothing works 100%.
First thing I would do is turn him out from the late afternoon to the early morning when the flies are less bothersome. During the day keep him in a stall with a fan. Best of you can mount the fan from above.
Next, get a good fly spray. I like the black ultrasheild. It seems to work best for me.
Fly gear. You can get a fly sheet cheaply from a tack swap or on FB. At the very least fly masks and fly boots are extremely helpful! Amigo fly boots aren't cheap, but they are worth it!
You can try a feed through fly repellent. I give my mare the Smart Bug-off. Personally, I think my mare looks more comfortable than the other horses who are not on it.
If you have horses at your house, or you have a small barn, you could suggest that all horse go on a feed through fly larvaside. That's also available on smartpak.
Lots of fly traps. I set the sticky paper one up all around my horses stall last year. I even made a home made one with a bucket, fly tape, dirty shavings and a liquid fly trap. I caught HUNDREDS, if not thousands, of flies!