I'm no expert on portraits and others will give you technical advice, if needed, on the lighting but I would crop the bottom about just below the neckline of the t-shirt. Far more serious, for me anyway, is the way the top of the head is sliced off. I dont have a problem with the back of the head being lost behind whatever it is he is leaning against. There seems to be a bit of a fashion for cutting heads in portraiture and I don't understand why.
The skin tones on the face are a bit washed out too. I like his expression very much; he looks a great little character.