Actually, if his parents were married, and what evidence exists suggests that they were, and he was born outside the US, he was not even a citizen at birth. He could have been naturalized later, but that would make him just an ineligible as being an illegal alien.
He is not obviously even a citizen. According to the rules as they then were, if he was born in, say, Kenya, then he is not a citizen at all unless he was actually naturalized.