Everytime my son catches cold and cough, his cough heals very late.please suggest an effective cough syrup or medicine?

The safest thing to : Do is see his pediatrician, as your son may have reactive airway disease or mild asthma. Upper respiratory infections can trigger bronchial spasms in people with these conditions, requiring medications called bronchodilators. It's always wise not to suppress coughs in children, except maybe at night for more restful sleep, but only if his doctor okays a medication with dextromethorphan.