He mentions “natural born citizen” in his CNN interview.
http://www.washingtontimes.com/news/2011/apr/10/trump-rides-issue-of-presidents-birth/
“Why doesn’t he show his birth certificate? The fact is, if he wasn’t born in this country, he shouldn’t be the president of the United States,” the billionaire developer said in a CNN interview that aired Sunday. The Constitution requires that the president be a “natural-born citizen.”
In that quote he focuses on where obama was born. Even if obama was born in Hawaii he may not be a natural born citizen. Trump shouldn’t box himself in. Obama may have been born in Hawaii (and hiding something) or is able to get officials to forge what is needed.