Except that he isn't human and, while found, clearly entered the US illegally. A lawyer would have had him claim refugee status. You cannot just throw a child over a border and have them "found" by someone. Foundling is a commonlaw principal to cover situations where it is not possible to determine a child's history and it is therefore assume they were born locally. It doesn't trump obvious evidence to the contrary.
Brev I think they'd make an exception for an alien baby found in an escape pod.
Two farmers in Kansas are going to go to the county office and say they found this baby, and simply leave off the "alien escape pod" bit. We, as readers, know that Clark Kent is an alien superhero, but nobody else does or needs to.
It is not possible to establish except you are superman and found the Crystal Palace. Not possible for the court and in fact any Human for quite sometimes before he comes to age.