Cough only at certain times?

Erica

My 2 year old has a horrible cough, it’s so bad that it almost wakes him up and it almost makes him sick. But weirdly enough, he is only coughing early in the morning when first waking up, or when he’s about to go to sleep, AND while he is sleeping it is at it’s worst. Anyone know what this could be? He has NO other symptoms.