The farrier solution is to put clips on the shoes. Clips will hold the shoes in place, thus preventing them from sliding around each time your horse stamps a fly. But perhaps a better solution would be to keep your horse in during the day when flies are active, or at least provide him with a run-in shed or other reliable shade. Your horse will probably appreciate this approach. A serious fly control program will help as well.
Flies breed in moist decaying organic matter.
Horse manure and bedding are a favorite with flies. Keeping your barn and paddocks free of manure will help tremendously. If your paddocks are too big to patrol with a wheelbarrow, you can drag them with a pasture harrow. This will break up the manure into pieces too small for flies to breed in easily, which will dry in the sun, making them largely uninhabitable by fly larvae. Fly predators can work minor miracles.