[Lmo56]

Does that only consist of the 50 states and DC, or are territories and overseas military bases included in that? For example, was John McCain an NBC based on your definition?

It generally means - that a person is born within the sovreign's realm. Exceptions being those children born abroad of parents who are serving abroad in the sovreign's name ...

Given the ease of travelling these days, however, I think that the Founding Fathers would have relaxed that definition to include children born abroad to any parents [in the sovreign's service or not]. However, the foreign residency could not be permanent AND the parents would have to be US citizens ...

[IMR 4350]

To the best of my knowledge, I don't think most countries grant someone citizenship simply by being born in the country, particularly if the parents aren't residing there.
The US is, to the best of my knowledge, one of the few that does. Even then you are just a citizen, not a natural born citizen.

In fact I don't know if all countries allow the mother to transfer citizenship. I don't know enough about the law, but it's possible Kenya doesn't even recognize him as an American citizen simply because his father is Kenyan and the mother was American. If Kenya doesn't allow the mother to transfer citizenship, then in their eye, he's Kenyan.