I think you can try treating your horses skin from both outside and inside. First of all, a good coat conditioning supplement, such as ground flax, black oil sunflower seeds, or a supplement that we like and does an excellent job on the coats and hoof growth is a granule made called Hooflex. The HOoflex coat and hoof supplement is expensive but we had the best results with that, horses just dappled right out. I dont like to use corn oil because it really goes to the neck of the horse and fat content is so much higher, but as far as the coat goes it helps.
Healthy Hair Care is a moisturizer for the coats that you mix with water and spray onto the horses coats. We use this right after washing, conditioning manes and tails, and rinsing them, get excess water off, then spray on the healthy hair care all over the horses body and mane and tail. It really helps to restore the moisture back in. Its sold in a concentrate that you mix with water, goes a long way and is a good product. MOst of the tack shops sell it.
You can use it right after you clip them too for moisturizing the coat, it helps to restore the color in my opinion as well.
A hot oil treatment will also help too. Some horses just naturally have a dryer coat and will take a bit more conditioning to get the maximum results.
When warmer weather arrives you can try letting your horse out when the sun is not on him as much, like early mornings or evenings and nights. That will help as well. Good luck to you. Laura