It sounds like crop, I agree with Trillian, the cough is worse at night. It is best to get the Prednisolone (Redipred/Predmix) in before the night again, it jsut aids int he recovery and brings releif overnight.

If he is wroking harder to breathe even during the day I would get him into the Dr's to have a look at his chest and listen sooner rather than later
Take care xxoo