I said yes, your son derives citizenship through you, provided that you have legal custody (even if it's shared custody with the birth father). As of the Child Citizenship Act, which became law in Feb 2001, only one parent needs to naturalize in order to pass citizenship onto their children. Before that, they used to require both parents naturalizing in most cases. So you don't need to worry about his stepfather's status. And it doesn't matter how your son got the green card (as long as he got it legally and without fraud, of course).