Is he an obvious foreigner? You mentioned that his mother is of Chinese descent, does he look really "white"?

If you can't speak the language and you are an obvious foreign looking kid, you will be susceptible to the usual scams and ripoffs.

I wouldn't want my kid to jump in a taxi or be out too late after dark - you just can't be sure. However if you are worried about physical violence or kidnapping, you shouldn't have anything to worry about. China is pretty safe in this regard.

Just use common sense.