My3 year old had bad flu about a month ago,and after that he developed a cough which has been going on for about three weeks,its worst at night.????

Dry cough: A ongoing sore throat warrants an examination of your Throat and physical examination. A post influenza cough usually is dry and due to irritation/inflammation of the trachea. It may be a long drawn out process before the dry,hacking cough clears. Of course your physician should evaluate you for persistent cough before dismissing it as merely an irritated windpipe for which robitussinDM ordered