Not an expert at all but I have a similar problem.
Itchy Ozzie has been mine for 7 years. He is a Paint/Quarter horse gelding with terrible allergies. He is itchy all summer long.
Things I tried:
Allergy testing – gets sublingual allergy medicine daily in increasing doses. This is helping a lot.
Antihistamines: tried hydroxyzine seemed to affect his personality and did not work very well. Now – he is on benadryl – available OTC without a script.
Fly predators – did not seem to make a difference.
Bathing – try Selsun Shampoo or tea tree oil shampoo.
Turn out – nighttime in the summer
fly sheets – worth trying but he hated that.
fly spray – who knows what works????
Topical treatment – tried baby diaper ointment, veterinarian’s cream, icthamol. maybe they help for itching and soothing.
Fans in the stall – good for less sweating.
Those are my techniques for trying to keep him comfortable.
I’m always looking for anything that will help him.