My horse gets spring and fall allergies..actually he's allergic to everything haha. So we get a feedthrough called Trihist. Its an antihisimine equivelent to sudafed for people. Anything and everything sets his allergies off. Hay or no hay. He even gets hives from rolling in the grass. I know. Sad. Lol. Just wanted to put that out there, he used to get really weepy eyes too, now when we notice him coughing or sneezing more than usual, we just give him the recommended dose for a couple days.
Btw, you can only get it through the vet, so they may have to come out and look at your horse before they can prescribe it.