Lots of causes.
It could just be a cold, with the cough
being caused by post-nasal drip.
However, if his temperature is over 102.0, if he's inconsolable or poorly rousable, or if saline nose drops and suctioning don't help with the cough, he needs to be seen today to make sure he doesn't have something more severe that requires additional treatment. I hope he feels better soon!