The first thing to do is improve the quality of his skin. Adding 50,000 IU Vit A daily to his diet as well as 4 oz (100g) ground flax (has anti itch and inflammatory properties) will help. If he'll eat more flax, give him more (mix it with a little grain).
Next I would evaluate his environment looking for triggers. Does he have this problem every year at this time? Is it allergies? You can test for environmental triggers (trees, grasses) as well as molds, dust... It can be a little pricey ($200+) but if you don't know what is bugging him it's hard to treat. If it's pollen of some sort, blanketing will actually aggrivate it by trapping the pollen against the skin, he sweats, he itches more...) Is his ventral midline raw? Have you checked for lice? If his blanket is off what does he really want to scratch? Has he been dewormed for pinworms and tapes?
Other things you can do is to give him an antihistamine. You can give a horse regular old Benadryl. Crush 10 pills (adjust for his weight) and add to his grain. Only problem it only last about 6 hours. If the itching is worse at say dusk, then give it so he gets the most relief at that time. Longer lasting steriods such as Dexamethasone are very cost effective and offer greater relief. (up to 2 weeks). Hosing him off daily can help too.