If it occurs before the election, he would be immediately disqualified (following the necessarily expedited trip all the way to the SCOTUS). The Democratic National Committee is empowered to name a replacement candidate, which would undoubtedly be Hillary! Clinton.
If he wins the election and this occurs before the inauguration, we'd again have the legal whirl, followed by the election being thrown into the House (which would most likely select Hillary!).
If he wins and is inaugurated, then Obama will be forced out of office (again after the legal stuff) and Joe Biden becomes president in accordance with established rules of succession.
My guess ... if this bears any kind of fruit at all ... is that it would become an issue post-election. Hard to imagine that it would be resolved within the next month.
The one "expedited" way for this to be quickly resolved is for a whistleblower in the Hawaii government to leak the vault copy of Obama's birth certificate, or to leak that there isn't one.
-PJ