Sure there's subtlety and complexity
*@ozanj* but there are also far, far fewer black and white people in Japan than there are black and brown people in the UK. Especially in large, racially diverse cities in the UK (which his 'me, mummy and the teachers' comment implies).
I've been to Japan. I'm white. We were objects of curiosity, followed around the streets by children. Especially my blonde friend. I was the first white person my Japanese friend's grandparents had ever met. They had met one black man, 20 years earlier. This in a large city, not some rural backwater.
What I'd want to know from OP's DH, is what he meant by his silly comment. Were I OP I'd have asked him. Demonstrating the calm demolition of a silly statement is a far better life lesson than jumping to conclusions and calling people names.
He could have meant 'there is considerable racial homogeneity in Japan, far more so than here', or he could have meant 'I actually think Japanese people all look the same'.
I wouldn't expect anyone to have the detailed knowledge of Japanese racial subtleties that you have but there is always the internet, to check things when you're not sure.
That 'he argued it was true' isn't looking good for him. Further, in my view (as discussed upthread), deliberately linking skin colour with culture and geography is not helpful and doesn't come naturally to children, unless they're taught to do it.
So if it was a silly throwaway comment and he then recognised it wasn't the cleverest thing to say to a child, that would be one thing (whatever he actually thought or knew himself). Digging his heels in about it being a suitable thing to say to a child is quite another.