I live in the Tennessee Valley, or "allergy purgatory" as my vet calls it. My horses and dogs never had skin allergies until we moved here 8 years ago.
One of my horses has dust/pollen/mold allergies and every spring I would put him on Tri-hist. It amped him up so much I started looking at things-herbal.
He has been on Allergy Herbal Blend : Build up horse immune system and help cough
for three years. This time of year I have to up the normal dosage but it works for him and actually "zones him out" instead of amping him up like Tri-hist did.
This horse is 16.1H and weighs 1,100# so the owner of the company said would not hurt to double the dosage. Because the stuff mellows him out so much, I give him 3/4 dose in the AM and 3/4 dose in PM. It is pelleted and he willingly eats it.
Within 48 hours of the first dose, his eyes and nose quit watering and he quits coughing unless I forget to wet his hay down at night.
It is pricey stuff but it works. You would just have to watch your horse for a few days to make sure he didn't have an allergic reaction to one of the herbs in the product --- anything is possible.
I also keep this horse hosed down every night in the summer and use a cheap hard plastic brush to scrub the day's pollen & dander off him.
If he shows me he is really itchy I will shampoo him with MalAcetic, also an all natural product for skin allergies on dogs, cats, horses, etc. "DermaPet" MalAcetic Shampoo for Dogs and Cats (ONE GALLON)
It can also be bought in an 8 ounce bottle. A little goes a really long way. I still have half a gallon left from two years ago and I bath my dogs in it and all four horses if need-be.
Diet plays a key role as well. If you are feeding sweet feed, try to find something "quieter" and get him off it. Different types of hay may or may not affect him.
Good luck and I hope this helps