This is a good type of piece to practice lightsources and etcetera with, and you did a good job laying out the shadows across the face.
I'd try smoothing out the jaggi-ness along his cheek (his right, our left) and other areas like that, maybe give it some AA as well.
also: the nose looks a little flat, try defining the nose-wings and giving the bottom edge a little more round-ness.