My 3yr old boy has been coughing for about 2weeks even after meds. now on expectorant for 2 days. any reason for the persistent cough? advice please.

Chronic cough: Not sure what meds were tried, but an expectorant is not going to help -it's going to increase the cough. At this age, i'd be concerned about possible aspiration of a foreign body (is there a lego down the airway), prolonged infections like pertussis (whooping cough), ge reflux, or asthma. All of these need to be explored by doctor instead of just giving cough meds to try and hide symptoms.