I have a 22 month old son. He is generally a healthy kid but he coughs a lot. It is mostly dry cough. I have tried humidifier, vaporizer, prospan coughing syrup. None have worked yet and is very annoying for him and does not let him sleep well. It used to be only at night but now with the lightest cold and running nose, coughing starts and looks like there is no end for it. Thanks for your help in advance.