In order to teach in a Japanese university a Master's Degree or higher is desired. There are Master of Arts degrees in teaching English as a Second Language offered by many schools; some are designed for people who are working abroad, like the School for International Training School for International Training (SIT) Home - SIT Home Page
I highly recommend that your son first pursue a Bachelor's Degree and get some experience as a volunteer ESL teacher, which can be done in any number of religious or social charitable agencies that serve immigrants. As for the major, Linguistics or Applied Linguistics can be good preparation for teaching a language. I also recommend learning Japanese, as once you leave central urban areas in Japan you really need some skill in the language to live there. Learning other languages can also help him to gain more understanding of the language learning process.
I taught in Japan for 7 years in total, one for the JET programme and six at the university level, when I returned to Japan after doing coursework for my doctorate. I did my doctoral research in Japan and taught there for several years after earning the Ph.D. Feel free to post any more questions that you or your son have.