Are you sure his diet is completley balanced, with all of his minerals? Lack of minerals can lead to dry skin. I have heard washing his mane with an oatmeal shampoo, and then applying conditioner, then eucalyptus oil into the roots of the horse’s mane to rid it of any fungal infection helps get rid of it. After that, every two weeks, give or take, repeat that process.
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown