How is telling him to travel back in time helpful?

The best way to learn a language that doesn't involve time travel is simple immergion. Watch movies with subtitles, pick up novels or short stories, anything that exposes you to the language. For this to work, you'll also need to find someone that knows the language, be it a teacher, a friend or anyone else. This person or people can act as a bridge and put what you're seeing into context. Taking a class at the same time will also be helpful of course. This is of course assuming you have things to do; if not, the easiest way is to just go to Japan and live with some bilingual family.