I know it looks gross seeing him covered in flies, but I don't think they can do too much damage. I'm guessing that the cut's on his head? I couldn't work it out from reading this topic. But if its only a surface cut over bone, they can't get in deep to lay. And besides, if they do, you will see it straight away and can get them off. Apoligies if that sounds gross!
Flies are mainly a problem for deep, fleshy wounds that go too deep for you to see exactly what's going on. I wouldn't worry excessively about flies, sure if you do have a way of keeping them off, go ahead but don't fret too much if you can't. Often there will still be some flies even when you do put on loads of repellent - happened to my horse with her massive (25cm long, 15cm deeep) cut.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare