Ah, I have a couple of hens who sneeze a lot and always used DE in the coop. I was told from someone in one of these forums not to since it can cause respiratory issues. I stopped using DE in the coop (I use PDZ instead) and the respiratory issues are gone. Also, I have a camera in the coop, at...
Any update on your guy? I just came across this post and his open-mouth breathing/sneezing could be some sort of respiratory disease. Is the bedding in your coop dusty? Dirty? Do you use DE in the coop bedding?
Vet Rx has helped my birds with respiratory stuff before.