Well, Bowser Jr. isn't a boss, he have his own minigames, the Bowser Jr. minigames, at his own space, the Bowser Jr. space. When you land on it, you will do a 2-vs-1 minigame with one player as your ally against him. (There's a total of 10 Bowser Jr. minigames in all.) You can see him on each board so this is not a boss.