His mother was a Mexican national when he was born, making him a Mexican national.
First of all, his mother was born in Delaware and his father was an immigrant from Cuba. He was born in Canada to US citizens although raised in Texas, which theoretically allowed him to declare dual US-Canadian citizenship, but he renounced any claim to Canadian citizenship long ago.
Second of all, all those defending the Alamo were Mexican nationals themselves! Texas was part of Mexico fighting for independence then. After winning independence it was its own nation for 10 years before joining the US.