Sorry if it's a dumb question, but what does getting a US passport for my son prove for him? basically just a good step towards filing N-600? Once again thanks for the help.
For most purposes, a U.S. Passport will be accepted as proof of U.S. Citizenship and is required for most foreign travel. A passport expires and can be challenged, DOS can revoke it if it is issued in error on its own authority and in its own discretion. If it is a complex case, the Passport Agency can make one get a Certificate from USCIS first.
An N-600 resulting in a Certificate of Citizenship is primary evidence of citizenship and does not expire. It can only be revoked pursuant to a challenge in a U.S. federal court and the government bears a heavy burden of proof.