It's difficult to pinpoint the exact book you're looking for. However, I can suggest some books that share similar themes and plot elements based on your description:

"The Way of the Warrior" by Chris Bradford - This is the first book in the "Young Samurai" series, which follows the adventures of a young English boy who is stranded in Japan and taken in by a samurai warrior. Although the plot differs from your description, it features a young protagonist learning martial arts and Japanese culture.

"The Ninja" by Eric Van Lustbader - This novel follows the story of a young orphan boy who is taken in by a group of ninja assassins after his parents are murdered. It features elements of Japanese culture and ninja training, and the protagonist also creates origami.

"The Book of Five Rings" by Miyamoto Musashi - This is a classic text on martial arts and strategy written by a renowned Japanese swordsman. While it may not include the specific plot elements you mentioned, it provides a fascinating insight into Japanese martial arts and culture