The US would not know he was a citizen unless his parents filed a CRBA on him when He was born. That is the purpose of that form, to regiater US citizenship for somebody who is born in another country. My parents had to do one on me when I was born and I had to do one on my kids born in Germany.
And??? The CRBA does not make one a citizen - it just records the event, like a birth certificate. Someone born in the US that is not issued a birth certificate is still a citizen. Someone born outside the US to a US citizen is still a citizen even if the CRBA is not filed. The filing just makes it easier to prove that fact.
The US wouldn't even know that the person existed! The US doesn't know of the existence until the person shows up and makes a claim of US citizenship. This presentation can happen at any time in the life of the person. If they never present the claim, the US never recognizes them as a citizen.