R.D.
Nebulizers are *amazing*. I bet your little dude feels better by tomorrow evening!
As far as what to look for in him potentially taking a turn for the worse, cuddles are the best remedy :) It makes you feel better (although super tired!) having baby right there, and gives them comfort. If his lips or nail beds turn blue, or he's literally gasping and choking trying to breathe, go to the ER. BUT, I think the nebulizer treatments will help him out a lot!
Don't be surprised if he gets really hyper after having a treatment, that stuff is like baby crack (not really, it just makes them super wild the first time!), so do it at least 30 minutes before bedtime ;)
Hope he feels better soon!!