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?