Sounds totally like allergy. Even people develop allergies later in life. I keep a fly mask on my horse during the day but you said this happened over night. You could try a mask during the day, take it off at night (never leave it on at night unless he is in the barn cause they can’t see out of it at night) and then stall him overnight. I don’t think they make allergy medicine for horses but you could check with your vet.