Like it or not, he doesn’t need to prove it.
If you don’t like that, help work to get state laws on the books that require any candidate for office (any office) prove his citizenship status. That’s the only solution at this point in the game.
Actually, he had to declare under oath that he was a Natural-Born Citizen when he applied for the Arizona primaries. That makes him liable for false swearing.
http://naturalborncitizen.wordpress.com/2009/07/27/obama-is-guilty-on-at-least-two-counts-of-false-swearing/