My son(3 years old) has not stop coughing for the past two week and the medicine we have is not working. any advice?

Not surprising : Cough/cold meds at this age have little if any value and sometimes prolong symptoms. Common URI viruses can trigger a cough lasting up to a month. As long as there is no fever & the kid plays/eats/acts pretty normal you can wait it out. If the cough seems to interfere with his activities it may be worth a re-check.

Go back to doc: Cough may indicate underlying wheezing. Give antihistamine calculated for his age & weight.