He is a citizen under Law. And that is precisely the point. A law had to be created by congress to make him into a citizen.
You need to leave out the word "natural". There is nothing natural about a condition bestowed upon you by a vote of men.
From the document that created US Citizenship:
When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.