Final answer:
The boy might draw his father inside a square to symbolize confinement or the feeling of being trapped. It could also represent the boy's emotions or the constraints in his relationship with his father.
Step-by-step explanation:
The reason the boy might draw his father inside a square could be related to the artistic choices of the painter. In the context of the given information, Henri Matisse painted an image of his son during World War I, when his son was mobilized.
The square could symbolize confinement or the feeling of being trapped. It could also be a representation of the boy's emotions or the constraints he feels in his relationship with his father.