Thank u mam for ur answer. My child keeps his mouth open very rarely and I had been to paediatrician and I was told my child is having allergy so I was asked to give him nebuliser and also cough syrup and levocitrizin I did all this but no much improvement.

A child's cough can sound terrible—and some may require a trip to the doctor—but most aren't a sign of something serious. In fact, coughing is a symptom, not an illness. You could also understand if he keeps mouth open and sleep. This could also be the reason