Yes, Ethnically Korean, his grandparents were immigrants. As for cultural differences, I think you could credit his extensive time spent living in the US as much as the immigrant Korean culture.

Japan has a weird split between Japanese nationals, Korean special residents, and actual foreigners (including "normal" Korean immigrants).

It's a whole can of worm I'll only leave here for any else to open, but Son Masayoshi was born in Japan from parents who might have formerly been Japanese (Korea was a Japanese colony) depending on the circumstances.

Many people in similar cases are basically undistinguishable from regular Japanese nationals culturally speaking.