What is your nationality on the dad's/Mom's side? But, generally, it is impolite to ask about race/nationality. It is more polite to wait till the other person breaches the subject.

I guess as a person of color you are in a better position to ask this question, assuming of course it's not with malicious intent: https://www.phrasemix.com/phrases/ethnic-background

I think I would wait for the other person to 'volunteer' the information. This is because I believe that mixed race people can get tired of people asking them about who they are 'really'. It certainly depends on the person, though. My children are mixed race and they would probably not mind being asked, for example. Still though, I think that if someone is getting to know you, they will tell you about their roots when they feel like it. Any other opinions on this? Ps - we can't say - what's your other half race. I would just ask ' where are you from'? This gives him the chance to say 'I'm from Japan' or to volunteer more information. It gives him the choice of how much he wants to share. My kids get asked this question all the time - because people see an Irish woman with Greek kids and they get confused or curious. My kids say we're Greek-Irish. Just like saying African-American etc.

"Where do your parents come from" = A standard and not rude way of asking your simple question. To which no one should take offence. Or if you want to ask indirectly for some reason. "Is it your mother or father that/who is Japanese?" <<--If you have not seen his parents. This will lead gently into then asking what nationality his father or mother his depending on his answers. You can then ask where the other parent comes from.