Just wondered if some antihistamine might help. If you are sure he is breathing ok, and asleep, perhaps you could just watch him? I would keep him in with you though. I guess it depends where you would have to take him if he got worse. Could you clean and reassemble the nebuliser now, while he is sleeping? Then at least it is ready if needed?
Dd had a wheezy attack the other day - ambulance and all. never had an attack before. Apparently a bug of some sort. 2 nights in hospital later, neb and steroids. 3 other kids at her school also had the same problem on the same night.
My ds2 had very bad asthma when he was small. The best piece of advice the GP gave me was never, ever stop his preventer, under any circumstances, and always increase it if he showed signs of an attack. The reliever on its own is just not enough.