Passport Question

OK smart guys, just booked a new Years Eve cruise for myself, wife, son and his soon-to be-wife and daughter and her soon-to-be husband (yeah, weddings galore coming up). Question is : soon-to-be son in-law does not have a passport. He was born outside of the US to Amercian parents. Holds dual citizenship. But since his birth certificate is from out of country what does he need to apply for a passport? Any help would be greatly appreciated. The cruise isn't until 12/09 (I'm a planner), so we have plenty of time.