D..
Croup is usually worse at night. If he starts it up at night, take him outside in the cool air. People panic and rush to the hospital, and it's better because they have taken their kids out into the night air.
Sometimes kids need breathing treatments with croup if it constricts their breathing (that happened to my 7 year old - I was astonished that a child that old would still get croup, especially as bad as he did.)
Watch and listen for wheezing and "off behavior" that signals he is having trouble breathing, since he can't tell you.
Good luck!
Dawn