Tracking down records of grandfather who lived in China under Japanese rule in WW2?

Now that I live in Tokyo, I’m wondering if I can dig into my grandfather’s childhood here as he lived under Japanese occupation.

For context, my grandfather lived in Japanese-occupied Xiamen during WW2 from 1938 to 1945.

Would the Japanese have kept details, such as a koseki (戸籍), on him and his parents? And if so, is there an archive here in Japan where I can pull those historical documents from?

I’m curious to know how he lived since he never really shared what life was like under Japanese rule, and would be nice to share with my family.