Looks like we're dealing with this too. Just took the mutt to the vet yesterday for coughing and vet thinks an allergen is irritating his trachea. The shot of steroid he got yesterday has done wonders. Hate the idea of a mutt who is a food hound on steroids. For some reason, it ramps up his want for food 1000%.
However, what I am curious about is what will testing him for allergies change for you? If it is something in the air, there's really nothing you can do about it.