I'm one of those parents who was woken in the middle of the night, threw her kid in the car, temp outside about 40 degrees, drove to the er and never noticed that by the time we got to the hospital he was breathing normally.
Both our sons get "croupy" from time to time. (A doctor once told us that that doesn't mean they have croup, so we just say they're "croupy.)
Our oldest (now 6) responds well to the hot, steamy shower and to being bundled up in the cold air. Sometimes drinking some cold juice helps, also.
Our youngest (3 1/2) pretty much needs the cold air. He gets so upset that he makes it worse. I usually hold him and try to get him to relax and drink some cold juice, but mostly I'm just stalling while my husband gets himself bundled up and a blanket ready so that he can take our son out into the cold air.
Definitely listen to your doctor. If this happens again, you will be so much more calm and will have a few ideas to try. My best piece of advice is to remain calm, try a couple of things and just see what works for your son. If he's anything like mine, he might sound "croupy" with every cold. After listening to myself during a cold once I wonder if my parents had to deal with this when I was little.
Good luck!