He may have been born in Hawaii, but the alone does not make him a Natural Born Citizen. A natural born citizen is a child of citizens, plural. Although in the old days that would have probably been stated as the child of a citizen father, which he is not, by his own words. This is because in those days a woman's citizenship was that of her husband, so that way of speaking still fit the old definition of NBC, as a person born in a country of parents who are its citizens at the time of birth.
Although since he refuses to say which hospital in Hawaii he was born in, even Hawaiian birht is somewhat questionable, regardless of what his Birth Certificate might say. (IOW, it might say he wasn't born in a hospital, which would be unlikely for the time and his mother and grandparents socioeconomic status.)
Ever heard of an “anchor baby”? If a child is born on American soil he’s an American citizen regardless of his parent’s citizenship.