I think M has croup.
My internet research is telling me that it's caused by a virus and there's no real treatment or need to go to a doctor as long as he's breathing ok. If he has trouble breathing, we call the doctor. Does that sound right?
ETA: In case anyone is interested, I talked to the pedi and she said they don't need to see him unless he gets worse and really has trouble breathing. They suggested running a humidifier and taking him outside to breathe in cold air if he starts coughing. Hopefully the little guy will get better soon.