What is the exact care for ear mites? My horse has sores in his ears that I first believed to be bites from horse flies but they are getting increasingly worse day by day. What would you recommend for the treatment of this condition?

Ear mites are not a concern for horses, however, other external parasites may be a problem.
Horses can be affected by several mites, lice and ticks. Will your horse let you closely examine his ears?
Ticks and lice can be seen without special procedures. To see mites, skin scrapings must be taken and examined under a microscope. This needs to be done by your veterinarian.
However, the problem may still be due to biting flies/insects. Some are very small and hard to see.
Try a fly repellent cream in the ears. Also, if your horse will wear a fly mask with ears, this will help rule-out/in biting insects, and possibly give him some relief. If biting insects are the cause, a mask should improve the ears in a couple of days, but it will take about a week for the ears to heal.
Also deworming with an ivermectin product will help to eliminate mites, lice, and ticks. If there is no improvement, your horse needs to be examined by a veterinarian.