What Causes Frequent Cough In A Toddler?

Hi doc, I stay in b lore & my son(3yr old) gets frequent cough every month.his coughing is more in the eve after he comes home after play.i am so worried since this is happening every month & I have been giving him soventus jr 5ml 2 times a day.it is hard as a mom to see ur kid suffer every month .pls advice