You’re saying that it doesn’t matter because the US government is able to prove his citizenship, but that isn’t in question. The crux of this matter would be whether OP was ignorant of his citizenship and if that ignorance would have any relevance to his case.

Securing official documents only available to American citizens makes it more difficult to argue that he was ignorant of his status as an American citizen. He likely could still make a compelling argument (provided he acts quickly), but it does make it a bit more difficult.

For digital copies of written works in the public domain, projectgutenberg.org is a good source.