People tried tracking him back in 2010, turns out some of the emails he sent originate from internet cafe's and public libraries in Tokyo. So the person at least physically resides there. I'd say there is a high likelihood he is of Japanese descent as well.
Why does perfect English have to mean he is not natively Japanese?
Considering his staggering expertise in cryptography, programming and mathematics I wouldn't say it's too far-fetched to think he is extremely well-educated, maybe even self-learned.
Not being from the UK or USA doesn't automatically mean you make grammatical errors, spelling mistakes or use poor language.
In fact, it means you're less likely to make grammatical mistakes, as you haven't used the method we appear to have in the UK(I odn't know what happens in the USA), "I'm a teacher, I won't teach you grammar, if you can't do it you should've picked it up already from reading!".