"(He/She is...) of mixed heritage."

I believe the current politically correct term is "mixed race".

I think the term 'biracial' is becoming quite popular. To me it sounds better than 'mixed-race'. i.e. He is biracial.

"This boy is of mixed culture. " "mixed blood" If you want to stress the bloodline. I would rather refer to this group of people as : "Those children whose parents (father or mother is) are not Chinese citizens."

So which one should I use then?