Actually, it's my understanding that his parents, or at least one of them, were from Cuba, not Mexico.
If I remember correctly, they were in Canada attending school, which is why he was born in Canada. So he has "derivative" U.S. citizenship--children of a U.S. citizen are U.S. citizens even if those children are not born in the U.S., as long as the U.S. citizen parent spent at least a certain amount of time in the U.S.
