I think the laws were different before he was born, where if one parent was a US citizen it didn't automatically make their kid a US citizen. It changed supposedly around the time he was born that if one parent is a US citizen then their child automatically is one as well.

Here's a summary of some of the laws.

https://travel.state.gov/content/travel/en/legal/travel-legal-considerations/us-citizenship/Acquisition-US-Citizenship-Child-Born-Abroad.html

Depends if his parents were married. Depends if his mom was in the US for so much time before he was born. The claim is he was born in Hawaii. Interesting how his grandma said he was born in Kenya, then died.