I know exactly what you are talking about. My OTTB, whom I've had for only a month or so now, has the exact same stuff all over him. The little scabs are rainrot, a form of dermatitis. From just a short amount of time my horse's skin has been looking so much better.
First, I put him on the Panacur PowerPack dewormers. It comes with 5 doses.
Everytime I ride, I curry him really well, use a dandy brush with the curry, and then top it off with a rub from the soft brush. My vet gave me these rags called "DermaCloths" and they have also worked wonders. I also reccomend an iodine bath or betadine scrub.
Always make sure to wash your hands everytime after you have groomed, and wash your brushes with soap about once every 1-2 wks. This makes sure you will not catch ringworm or spread it, because ringworm is commonly connected to dermatitis.
Hope this helps,