If born in the USA, then the citizenship of his father doesn’t matter. That was the point in WKA: that citizenship comes from birth in the country, and that is both NBC & 14th Amendment at the same time.
Here is a link to the WKA decision:
http://www.law.cornell.edu/supct/html/historics/USSC_CR_0169_0649_ZO.html
Here is the argument the state used to try to convince the Supreme Court that WKA was NOT a citizen:
http://www.scribd.com/doc/23965360/Wong-Kim-Ark-US-v-169-US-649-1898-Appellants-Brief-USA
I can’t provide a link because I haven’t seen it electronically, but last Saturday I read the decision regarding WKA that was appealed to the Supreme Court. The lower court rejected the rather novel argument that the wrong standard of citizenship had been used for our entire history, and that the Roman idea should prevail instead.
The issue of eligibility for POTUSA is more than just plain/ordinary ‘citizenship’. The Founders explicitly added the requirement for POTUSA to be a ‘natural born citizen’ and they intentionally did this as shown when they only required congresspersons to be just ‘citizens’. All following USA SC decisions are in the original framework set by the Founders.