Well he might be, but being a 'Citizen' is not enough, one must be a "Natural Born Citizen". But, if Barry was born outside the US, to a foreign citizen father, and a mother who was not yet 19, then, under the law in effect at the time, he's not a citizen, unless later naturalized. But of course that alone would make him not a natural born citizen.
Depending on how one defines "natural born", he still might not be even if A) born in the US, or b) his mother had been 19. He'd definitely be a citizen in either of those cases, but not necessarily "natural born". Unfortunately the Constitution does not define "natural born", so one must look to the historical record to find what it meant in 1787, when the Constitution was written, not to current law.
Amended certificates of birth may be prepared and filed with the Department of Health, as provided by law, for 1) a person born in Hawaii who already has a birth certificate filed with the Department of Health or 2) a person born in a foreign country.
Late registration, registration one year or more after the date of the event’s occurrence, of certificates are permitted subject to evidentiary requirements.
Has 0bama's birth certificate been amended? Probably. That's why we want to see the original.