My 4 Yr Old Son Is Suffering From Cough And

My 4 yr old son is suffering from cough and blocking nose.its coming frequently. So doctor gave montair sachets, xyzal.does my son have allergy.is this allergy symptoms .how can i know that he has allergyvand what kind of allergy.can this be cured or reduced.will it move to asthma related issues.how can i stop it from moving to asthma