You can contact the Doctors office or you can call the city that issued your son's birth certificate. The County Clerk usually does the birth certificates so you can call directly up to the Courthouse and they will fix it.
You will need to check with the hospital your son was born at. They will have an office that issues reprints of birth certificates. That same office will provide you with a form to amend the birth certificate and reprint an original with the corrected date of birth on it.