>>149281879 (OP)8 U.S. Code ยง 1401 - Nationals and citizens of United States at birth
The following shall be nationals and citizens of the United States at birth:
(f)a person of unknown parentage found in the United States while under the age of five years, until shown, prior to his attaining the age of twenty-one years, not to have been born in the United States;
So it depends on the circumstances of his arrival and when someone got access to the Kryptonian data files.
Otherwise he'd be eligible for a O-1 or EB-1 visa, because of his "extraordinary ability in the sciences, arts, education, business, or athletics through sustained national or international acclaim"