If the doc and the lady at the health department were correct in their analysis, either guy but more probably the second guy could be the dad.
Do you mean the doc said your due date was September 14? Or do you mean you thought of this yourself and the doctor said "Oh, yeah, sure"?
Either guy is within range if your due date is totally correct, though the second guy has better odds than the first guy. How did you get your due date? If from an ultrasound, how early in the pregnancy was the ultrasound?