http://en.wikipedia.org/wiki/Naturalization_Act_of_1790
"The Act also establishes the United States citizenship of children of citizens, born abroad, without the need for naturalization, "the children of citizens of the United States that may be born beyond Sea, or out of the limits of the United States, shall be considered as natural born Citizens"."
If his mom was a citizen, he is considered a natural born citizen no matter where he was born.
Superseded by subsequent law.