Too bad that he never obtained a certificate of naturalization. This comes up a lot. It's not late to apply for the future.
For now, bring all the supporting documentation, including US passport and copy of mother's naturalization certificate. Do not volunteer it at the interview. If you can proof his citizenship by using passport, he should use it. If asked about naturalization certificate, present all evidence you have, explain the situation and perhaps show a copy of filed application for certificate.
This is not a legal advice.