While the Japanese did not show themselves to be anti semites in particular, you can read up on some pretty horrible stuff they commited against humans. The judge there says that he believes that the boys did not know about it. It was a well represented case, and few doubted it. Nevertheless, they were sentenced.
That being said, an attempt at boycotting them would be counter productive. They would not suffer from your efforts, and they would only they it more so as a world wide jewish conspiracy.