I would get yourself some betadine scrub (this is a soap), Bathe your horse in this two to three times a week for a couple of weeks. Making sure to really lather it up in the base of the mane and tail and any problem areas let it sit for 10-15 minutes and rinse well. Betadine is an antibacterial soap and should get rid of most any bug. Also, you can start feeding about 2 tablespoons of corn/veggie oil at each meal over top of his hay or grain. This amount will slick up his coat and help get rid of the dry skin. Feeding over this it will become a fat source and the horse will start to put on weight.

If you don't see an improvement then contact your vet to make sure there is not something more serious going on.