I believe this is a duplicate question.
Depending on how old your son was when you became citizens (and whether you had custody of him at the time) he may have automatically been granted citizenship and you would now need to apply for a passport and certificate of citizenship.
You should consult the other answers that were provided to you on your initial post.
No, no N-400. Given that your son is still minor when either one of this parents became citizen, he is deemed to have automatically become a citizen himself, assuming he was already here on a green card, just like his parents before.
You thus need to file Form N-600 Certificate of Citizenship on his behalf.
I strongly also recommend that independently of the N-600 you take your son to the nearest US passport office and file for a US passport (even before you file for the N-600). You will see, he will get it! But also file for the n-600.
Behar Intl. Counsel 619.234.5962 Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.
What needs to be done depends on whether he had a green card when you became US citizens.
You should retain an experienced immigration lawyer to review all the facts, advise you, and handle the case. You can find one through http://www.ailalawyer.com.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.