If he's not welting up, there is no extra heat in the itchy areas, he's on a good deworming schedule and getting good quality feed, you may just have an exceptionally itchy horse like my 2 year old.
My boy was covered in dandruff when I got him in March and had rubbed half his mane & some tail out. I've never seen a horse so itchy that he will stretch out and nearly fall over during grooming sessions! I use Healthy HairCare Hair Moisturizer on him after every groom session, focusing on really working it into the roots of his mane and base of tail. If I go more than a few days without applying it, he will start rubbing raw spots on his chest and tail. Healthy HairCare is a pink liquid concentrate that you'll need to mix with water in a spray bottle; most bigger tack shops will carry it. It's comprised of various moisturizers, no drying ingredients like silicone. Took care off the dandruff & excessive rubbing, and leaves his whole body shiny and soft.