Thanks for putting up your query on HealthCareMagic. I am sorry that your 21 month old son is suffering from cough. Cough or coryza in medical terms is basically an allergic reaction that is triggered by agents like dust or viruses like respiratory syncytial virus (RSV) or certain other viruses that are always around us. Our respiratory secretions have something called sol-gel ratio and normally get mobilised upward on its own but when this ratio gets distubed and more gel-like, we call is cough and it is no longer able to be moved up spontaneously and needs to be coughed out. When it is even drier, we refer to it as dry cough. The solution would be in making it more liquid and less gel like. This can be easily done by two things. One is to avoid anything that you might feel him to be allergic to. Secondly, make the child drink more water. Thirdly, give him steam inhalation which is basically water in the vapour form that can go deep inside the lungs and liquify the gel-like cough. Doing this on regular basis two or three times a day is supposed to give him relief. In a process called nebulisation done in a hospital to relieve such patients, a similar thing is done with saline water with some medicines called bronchodilators. I cannot evaluate the requirement of bronchodilators without actually listening to his chest where I would have also been able to look for stridor or wheezing. Your paediatrician may be able to do that. Since you say that the chest seems clear to the doctor whom you have consulted, I assume it not to be some bacterial infection, croup, tuberculosis or whooping cough but lets not make things seem more complicated for you.