The off center positioning contributes to the picture's feeling of being slightly out of kilter, or moody, or melancholy - all emotions a viewer might project onto the picture, based on the look on the boy's face and his body language.
The window in the top left corner represents a potential design problem. Placing a significant object at the edge or top of a composition runs the risk of leading a viewer's eye right out of the picture world. In this composition the saving factor is the diagonal crosshatching of the window pane. The diagonal lines lead the eye back to the interior of the picture world, and encourage us to study the boy one more time.