You need to prove your child's citizenship.
Do you have your child's birth certificate?
I have it.
what documents can I use as proof of his citizenship?
You could use documents like his U.S. passport, Certificate of Naturalization (N-550/N-570) or Certificate of Citizenship (N-560/N-561).
And for me, do I need to show proof of identity, and what if I can't get a required document?
Yes, you also must prove your identity. If you can't get any of the required documents you'll get asked for another suitable document, like your employee identification card.
Can I use an expired document?
not, it must be current and show all the necessary identifying information, including a recent photograph.
Now, what if my son lived outside the US for a long time?.