I'd definitely take him to the Dr today.

Croup has a very distinctive sound. The cough sounds like a seal barking! DS had severe croup five times last winter, and the first, I had NO idea about croup.......but I knew what it was straight away because of the distinctive seal bark cough (which I read about on here thanks to SarahH ). Croup almost always attacks overnight, so I think you could probably safely rule it out in this case.

Given that he's been okay overnight I wouldn't stress about going to hospital, I would just get him into your GP today. Good luck!