Since I am against the idea of culling, (though I understand why under some circumstances it is the best thing to do) I would say continue to quarantine him and possibly give a little bit of antibiotics or aspirin. I have a chicken that would not stop sneezing for like a week. Gave her a bit of aspirin mixed in with her food for the day, the next day, the sneezing stopped. A few days later I put her back in with the flock, none of them seemed to have any issues. Yes, I'm aware this could be purely coincidental, but it never hurts especially when you mix it in with some yogurt to help make more they don't get an upset stomach. Hope that helped!