It won't go anywhere. Japan's police and legal system is extremely corrupt. The political system is also corrupt. It's not like the garden-variety corruption that people constantly bitch and blow out of proportion in the west; it's the real deal. Often this surprises people because they assume that because Japan is technologically advanced this must mean these aspects of their system should be "clean," but they're not.
If you get arrested for something and they decide to detain you, police can detain you for up to a month. During that month they can constantly interrogate you to try getting a confession out of you. You can't bail out, you can't call anyone and you can't have a lawyer present, to put psychological pressure on you and make it more likely you confess. If they indict you I believe the maximum they can detain you is 1 or 2 years. In those cases the interrogation can potentially go on for years. Let's say you're indicted for shoplifting, if you do not confess you're essentially guaranteed to serve at least one year in jail. A lot of people will prefer to confess just so they can get out early, as the penalty for theft is low. This hostage justice system is how Japan has a 99% conviction rate.
Police tend to target foreigners specifically. Just being non-Japanese is a homing beacon for police. They'll stop you to ask you what you're doing, where you're doing, where you're from, etc, for no apparent reason. Nothing bad will happen to you, it's just annoying. It's not impossible that black people get a little more harassment, but discrimination against foreigners is routine in every fact of life in Japan and nothing ever gets done about it. He's mistakenly assuming that skin colour is what's doing it and not the fact he's a stranger, also mistakenly assuming that the court system gives a shit about his lawsuit like they would in western countries. They don't give a shit, like at all.