His sperm donor was a British citizen, thus under the constitution, he is not "natural born".
We do have the problem of the framers of the Constitution not laying down a hard and fast definition of "natural born". Considering their absolute distaste for things British and indeed European, I would venture they had no intention of anything but two natural born citizens being the parents. If not that, two parents being citizens of America at the time.
The case of Chester Arthur, the Vice-President who became President on the assassination of President Garfield does raise questions. However at the time of his inauguration Arthur' father had become an American citizen. Thus his American mother and William Arthur were both citizens at Chester's ascent to the Presidency.
Since a number of British children were born in India and indeed China, they were considered British. True, one could say that American law thus superseded this. Perhaps it is thus written down somewhere.
A final note though, since the "birthers" are not crying "fire in a crowded theatre," there seems to be some sort of Alinsky afoot to deny them.
Free speech rules.
Our constitution is not a bare document. There is a huge paper trail discussing the nuances of every sentence.
Natural born means both parents born here as citizens. That is why they had to exempt themselves. They didn’t qualify in most cases.