Why does Gon spend thirteen years in the foster care system in the novel, Almond?

When Gon is four years old, his mother loses him at an amusement park. For 13 years, he is transferred between foster homes and youth detention centers. Then one day, a shelter calls Professor Yun announcing that they have found his son. Though he takes Gon into his home, Professor Yun disapproves of who his son has become. Gon hates Professor Yun, refusing to even call him his father.