I have a couple of horses just like the OP’s. I’ve found that if I can rinse them with water mixed with a few glugs of vinegar, it either eliminates the “hives” or greatly reduces the severity. I have an old OTTB that gets rain rot very easily even if I use the vinegar mix. With him I put Equiderma over his topline and the rain rot heals up and the hair grows back quickly.
Assuming your horse is not a sedentary pony, based off the analysis of your feed; he’s probably slightly deficient in a few vitamins and minerals. Giving him either a pound of ration balencer or a vit/min supplement. I’ve heard great things about spirluna, and my horse gets 10 zyrtec for any type of allergic reaction that’s bothersome, so assuming it’s legal, I’d try that (after a discussion with your vet) . General dosage guidelines are 1 10mg zyrtec per 100lbs of horse.
Permoxin. It is cheap. It is mixed with water. It can be used on Dogs. It CANNOT be used on dogs. I have used it for over 30 years and never seen it burn any horse. Bute I have heard on forums can burn horses.
You mix 10ml with 400ml of water. You can use a soft brush and brush it all over the horse. You can use a sprayer and spray all over the horse. Don’t forget brushing up the top of the neck into the mane and under the dock. Allow the horse to dry and put your day rug on .
If down to blood use 3 times a day. Working down to twice a day, once a day, once every 2 days, once every 3 days and if you are lucky once a week.
Flax oil and benadryl. I have a gelding with fly allergies. Put him on flax seed/soybean oil this spring. Not nearly as many issues. Skin and coat are 100% better, less scratching, itching, sores. When he does get itchy, I give him benadryl to get through the worst of it.
Antihistamines in general are forbidden substances
remove soy from his diet.
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OP, I’m in NOVA and feel your pain about rain hives (and hives in general). It takes VERY little for my TB to get some major skin issues, and we also have the same problem - they fade to scabs. When that happens, I usually need to bust out the SMZs, and Malseb shampoo helps, too (though not too often - will dry his coat out and make it worse). I’ve had some luck with Microtek shampoo too, but you need to watch out what else you put on them or you can cause…hives! 
It doesn’t play nicely with a lot of other shampoos/scrubs.
I find vinegar helps, but sporadically. He’s just a funky-skinned little guy and always has been. Sometimes a couple of days in a row with vinegar dries out his coat and we’re back to flaking and itching.
He is on flax, eats TC senior, I give him Benadryl when he needs it, he lives on Zyrtec here and there (usually spring only). His coat is beautiful until it’s…not. And usually it involves rain, or rolling in something. He wears a flysheet well, but like you, I can’t leave him in that in the summer…too hot. I have had good luck with Eqyss Mega-Tek Rebuilder. It’s a coconut-smelling paste that soothes the hives and regrows hair VERY quickly. Might help, and definitely doesn’t trigger any allergic reactions in my guy.
I’m mostly just commiserating. My horse is retired…my going boys don’t experience this, lucky for me. Good luck! It’s so frustrating for them and us.
Urtica urens 30c, 10 pellets 2x day. You can get it in the homeopathic section of whole foods, but its much cheaper online. I was SUPER skeptical of homeopathic stuff (still am, really), but after months of hives that would come back as soon as we stopped dex+banamine, I was willing to try it. This came recommended by a very highly respected vet in the Boston area.
My horse has seasonal allergies and gets hives. I have found that if I give him 1 scoop of Phyto-Quench by Ukulele and a tablespoon of Quercetin a day they go away. I get the Quercitin in bulk, 250mg from Pure Bulk. When the hives first show up it takes about 10 days of daily doses for it to work. After he clears up, if he gets them again, it only takes about 3 doses to make them go away. I make little packs so I can just throw it on his grain. No steroid or drugs, so nice. I hope this helps your horse.
I had a chestnut that would pop hives most every summer. I am thinking it might have had to do with corn pollen as there was a corn field next to her pen.
I found spirulina helpful. You can get powder (very messy) or tablets from Springtime. I had to break up the tablets but if the pieces were small enough she would eat with her supplement (ration balancer).
Susan
