Try grooming him very well and use spray after you brush him every day I do this with every horse I have and there coats are barely dusty even if they roled and they are super shiny!
Using a towel or a rag works uper well
Absorbine's Santa Fe conditioning spray works wonders!
Using a very soft brush helps get some dust off.
I like to use an Avon product called Skin-so-soft. Farmvet carries it. Or you can use a similar product. But I add it to a bucket of warm water, and soak a small towel in it. Then I curry the horse. Then I take the towel and rub the horse down with it (wring it out first). Then I take a large dry towel and dry/smooth the horse's coat out. Or the Lucky Braids Whitney spray work well if you just want to spray in on a towel and wipe the horse down with it (after fully brushing of course).
Rubbing with a damp cloth can help!
Scrub with a curry comb and then use a finishing brush