Ask him why there should be black people in it.

If he asks why there shouldn't, ask him again until he actually answers.

If that answer is anywhere along the lines of "I'm sure there were some blacks there at the time", ask him what time was it, and where were there blacks.

If he then tries to claim that because there were blacks in other parts of the world outside of Africa ask him where and how they got there.

If he's dumb enough to fall for the trap and say America and slavery, then ask him if he's suggesting Japan, and any other parts of Asia took slaves from Africa and watch him maybe start to realise he's fucked up. This is unlikely, though.

Keep asking him about every single stage of his stupid argument until it gets to the point he's basically two steps away from the actual reason of the slave trade in Africa then drop him in it and ask why would there be blacks in Japan again.