What Causes Persistent Cough And Running Nose In A Child?

my son age is 4 years his weight is 15.6 kgs. from last 16-18 months every month he is sufferring from coughand than it converts into running nose and than continious dry coughing and his eyes watering,under eyes skin also darkned.IS that he never get rid of this problem what do do?what prequations should I take?what is this disease.Please give a solution.