Studying Japanese for fun is pretty intellectual. Maybe he finds short cuts because he's bored in school. Since he's popular and happy, you could find academic opportunities for him on the weekends like a Japanese class or ask him about which subjects he likes without homeschooling him or asking for much of his free time to be dedicated to formal classes. He could read books in Japanese and get into literature that way, or learn about how different movements covered in English class (or high school English class), such as romanticism, are covered in visual art, since he likes drawing.
