My guess is that Trump figures that no matter what he finds out about Obama’s birth records, Trump wins and Obama loses for allowing this doubt to cloud his bona fides for so long. What was the purpose, if indeed, Obama was born in Hawaii? Obama is going to have to explain and he is going to look like a fool, or an impostor. Either way, Trump win.
He hasn't explained yet. Other than making a snide comment about pasting his BC on his forehead. Who is going to make him explain? Obama ignores, ignores, ignores.