I have a horse with sweet itch and for some reason he's also extra attractive to other flies. We have a few horses out together and while they might get a bite or two, this guy will literally have a swarm of biting flies around him.
This summer we tried just about everything. Garlic, fly sprays, different pasture, special shampoos etc. While some sprays worked, most of them were useless. The only one that worked well was 'carr & day & martin', but it contains high amounts of deet and only lasts about 3 hours before the flies are on him again.
We bought him an Amigo fly sheet and the relief was instant, long lasting, chemical free and cheaper then any other remedy in the long run. I was also worried about overheating aswell as we never used a fly sheet before but it it actually helps cool him down in hot sunnny weather as it reflects the sun light.Its a very fine, light mesh. This was a big bonus for us as this guy is black and would roast on hot days. Now he's cool and protected.
We also bought him a fly mask which was great as he was rubbing bald patches on his face.He was also sitting on the ground and rubbing his belly off his rear hooves, so there was bald patches down his belly aswell. We tried a variety of creams and potions and lotions on the bald patches to help them heal but none of them worked. In desperation one day i went out and attacked him with a tub of sudocrem (nappy rash cream) and he stopped trying to scratch within 2 minutes. Within a week the hair was growing back rapidly and now, about a month later, you would never know the bald patches were there. This worked remarkably well but if you're going to try it, just try a small patch first, and the only downside i found to sudocrem was that i had to apply it every day but i didnt mind as the soothing effect it had on his itchy bits was amazing.
The only thing i would say to you about the fly rug is to make sure he is fenced in with electric fence, away from anything he can scratch off. Might not be a problem for you if he doesnt have sweet itch but i learnt this lesson the hard way when i forgot to turn on the fence in his paddock and came out the next morning and found the rug at the bottom of the field. I couldnt even tell which end was which. It was just a rag with a strap here and there.
Sorry about the novel. I just felt for you as soon as i read your post. Its horrible to watch them suffer and hopefully some of this helps.