You don't. Japanese is an extremely difficult language to learn, so if it's going to be learned, the motivation for doing it has to be intrinsic - they have to genuinely want to learn it themselves. Maybe you'd be able to get him motivated to do so by trying to get him into subtitled anime or something, but that would also require him to want to explore that area himself.
I think learning new language (with very different fonts, pronunciations) could help to open-up brain & personality development.
High school age is well past the point where the brain and personality development are likely to be influenced by learning a language. Ideally, if you want to teach a child a language, you want it to be done as early as possible so that it hooks into the natural language-learning process of early childhood - and with a language as complex as Japanese, you probably don't want to seriously attempt this unless you're either Japanese or you've spent considerable time (years) living in Japan, because odds are, you'd wind up teaching it to them wrong.
Also learning about ancient Zen etc culture will help to bring calmness & focus in busy and chaotic student & university life
Now this is more doable, though I think you're also committing the sin of Cultural Appropriation. Again, unless you've got access to genuine Zen teachers, it's likely that any "Zen" you teach your son will be a bastardized New Age version that barely resembles the original, which is a part of a culturally significant religious belief system.
If all you want is some calmness and focus, just teach him secularized Mindfulness Meditation, instead.