Outlawrididge is so right on target with this.
I would keep him as clean and dry as you can. Sunlight is very helpful for it. If he's rolling alot, then lightly brush him off as soon as you can. If some hair or scabs come off, then treat those spots right away. Keep him inside when raining. I'm not a vet, but this is what I have learned, and what worked, when having to help treat it.
Rainrot is such a pain which is caused by bacteria that lives in the soil and it loves to present itself in muddy and rainy and humid conditions. After you later him up real good and leave the suds on him for a few minutes and bath him in something wonderful, keep this boy dry and out of the rain, away from bugs best you can; that's the key. Bed him good on clean shavings and if it's hot, run a fan on him. If you put a sheet on him, he'll sweat under it and that is also no good because the air cannot get to his skin easily. If he is hairy, shave him off so you can get to the problem easier if you are in a warm climate. There is also a product on the market that I have used for a million years by Farnum called Super 14 and Super 17 which is wonderful for bringing in gorgeous new hair coats on horses that have previously had skin problems. I highly recommend it; goes in the feed and they love it.