He renounced his US citizenship at 2 years old. Pretty thin.
Actually all he would had to do was renounce it before he lost his Kenyan Citizenship on one of his many trips there.
As has been pointed out to me in various COLB threads The US does not recognize renouncing your US citizenship if you continue to use a US passport like in the case of Marc Rich who became a Spanish Citizen and renounced his US citizenship to escape justice here in the US.