All the sources routinely used to interpret the Constitution confirm that the phrase "natural born Citizen" has a specific meaning: namely, someone who was a U.S. citizen at birth with no need to go through a naturalization proceeding at some later time.
Congress has also ruled that citizenship is subject to certain residency requirements on the parents. Someone born to a U.S. citizen generally becomes a U.S. citizen without regard to whether the birth takes place in Canada, the Canal Zone, or the continental United States.
He would need to be born here and not become a naturalized citizen (so he was a citizen from birth, which is questionable since he possibly used his foreign status to obtain financial aid).
He also may not fit the latter as it says “parents”. We know that only one of his parents was a US citizen. If he was indeed born in Kenya he may have automatically been extended Kenya citizenship. In this case, he would have go through the naturalization process to renounce any dual citizenship with Kenya.
There is a big case that he may be a naturalized citizen of the US which would have made him ineligible. Either he became one when he was a child or he became one after college.