The memory thing is pretty simple. It's not like his brain was completely wiped clean, because one of the first things he says in the very first chapter is "This place doesn't look like Japan." So he clearly remembers what Japan is like, and has this vague sense that that's where he should have been located.
Basically, he's lost any memories regarding his identity. So things like Japan and rice are familiar to him, but he doesn't have a personal context for them. He also retains practical skills like reading and writing and appears to still have all of his education intact. He still has the ability to socially function as an adult. He knows that the thing floating in front of his face is a "status screen" and knows that it functions like a game, meaning that he retains knowledge of modern technology. What he doesn't remember is anything related to who he used to be and what he used to do.