Was he?
He says he was, but seems oddly unwilling to disclose his birth certificate.
I believe the real issue is that his Kenyan grandmother has said she witnessed his birth in Kenya. That is what needs to be disproved, and why people are asking to see his birth certificate.
Even if he was born to an American mother in Kenya, he would not be a “natural born citizen” as required to be a president by the US Constitution. A “citizen”, yes, but not a “natural born citizen.”
“...seems oddly unwilling to disclose his birth certificate.”
Maybe it has nothing to do with citizenship; perhaps parentage.