I can tell you this: moisten the hay, use useful plants such as buckwheat, marshmallow, garlic, green anise or even essential oil of hop. Theses are antihistamine plants and natural ways to help your horse.
Then you can use meds such as cortisone injections in case of big crisis, shake no more product, or rhinallergy (homeopathy).
Put a mask on his nostril to limit the allergy, put him on a non-oriented paddock or even let him in the stable. You can also try desensitization, it is done during 3 years but costs a lot of money.
Good luck with it and don't hesitate to ask your vet for better advice :)