If one of his parents was an American citizen, he is a citizen...period. It does not matter where he was born. The reason it is so offensive is not that he's not a citizen...it's that he is a LIE.
I know that, I just like to think he didn't receive his elementary education here. It makes me feel better about our education system. I also don't think I'm the first person to refer to him as a kenyan.