actually, although it is not listed in IMDB, the boy, Kai Inowaki, has done many TV shows and also another feature film. it seems that he can play the piano little bit but not in the level of a piano prodigy. so no, he did not acttually played the piano in the film. he was "finger synching", which he did amazingly well. he is a talented rising actor, there is no question about that.
Surviving co-producer, Yuki Kito, spoke at the film's opening at the Music Box in Chicago and answered this question. The child was a piano student, but not as good as the film required. He was told he would have to practice this piece ("Claire de Lune") for visual effect, at least. They decided to use another 12-year-old pianist for the sound track. Ms. Kito said it was a good match.
Still, the kid who mimed playing the whole piece did very well, indeed! He was shot in close-up a few times, as well, and one not have guessed that he was anything other than really well-trained, so .....
I suspected that too. The synching at the beginning wasn't perfect, and they never really closed up on his fingers. However I am still surprised a 12-year-old can produce a performance like that. Even though the piece is not so technically demanding, the phrasing requires a lot of confidence and maturity..
Why do you call him "surviving co-producer"? Did the other people involved in the production of this film all die in some horrible accident? I'm confused.
If you watch the extras on the DVD, it was actually CGI that helped him play the piano. It was interesting how they explained the effect. If you look closely, it seems a little off but what they did was combine the actor with an actual pianist and combined the arm movements together. If you listen closely and watch his fingers, some notes are being played when he's not pressing the keys. Nonetheless, that's not the point. The point was that the character was playing the song.