No, I am not sure, but if you look at all the evidence discussed by other people who have researched his posts, there are a lot of signs that suggest he is a native speaker. If you sit down and read all of his bitcointalk posts, it's pretty easy to come to the same conclusion.

There's also always the chance his parents are Japanese but he was born in an English-speaking country, but I think considering the fact he clearly wanted to hide his identity, he probably picked a name that was unrelated to his true nationality and ethnicity.

He uses a bunch of colloquialisms which kinda hints at him living in an English speaking country. Obv one could also pick those up online, but I'm guessing that the first scenario is more likely.