I see MD readers had lots of money to enter the investment world with back when they were in high school. Maybe the dude has some savings, like let's say 50,000 yen, but he would need to remember specific companies that had their stocks skyrocket during those very years to multiply his savings to levels that would allow him to make significant money. As it is, he's more likely to gain more with a part-time job. Of course if he did those and invested all that money, it would be the win-win situation. Naturally if he could somehow affect how his parents are investing, the gains would be on another level. If he could make his parents invest millions of yens in certain tech companies, years later the profits would be jolly good.
Edit: If he was a savant and remembered correct lottery numbers from those years, he could be a yen billionaire instantly. But I doubt this is that sort of story.