Kids have a mentality that what they heard they will catch the things instantly, specially from the closest one.
It is good to know that he can continue with school kids with dutch and chinese with parents, he probably knows how to handle.
But he will be confused when meet someone new as which language to choose.
But yeah, in the long run, his experience let him be more helpful in touch with people.but at this age things will messed up a little for being confused some while.
it is happy to know that he can select the right language to the right person, he is surely a quick learner and smartest one.