If there are no scabs and bumps and it is on the side of his neck, he just has lice. They get real bad this time of year (just like rain rot) because the horse has run out of all Vitamin A stores.
You can treat rain rot and lice and they get all better about the time they shed. Or, you can just give the horse a big dose or 2 or Vitamin A and feed a supplement or mineral that has a lot of Vitamin A in it and they will get rid of it much more quickly. Feed them Vitamin A all hrough the winter and they will not get rain rot, goopy eyes, lice and scaley skin -- all of which are sypmtoms of a Vitamin A deficiency.
You can give a horse 3 or 4 cc of an injectable Vitamin A medication found at any livestock supply store or catalog. You squirt it in the horse's mouth and do not have to inject it. You can repeat once a week. If you feed a supplement, use one like Mare Plus made by Farnam. I have supplemented Vitamin A for over 40 years, particularly in the winter, and have not had to treat rain rot or lice since and have not had to 'clean' a mare since.