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 need to obtain a legal change form form the jurisdiction in which the birth certificate was filed. You need to consult them as to papers you need to present.
Contact the city or county records department to get a new one with the correct date after showing them the hospital records that have the correct date of birth on them.
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.
