Just sharing my thoughts and impressions...here it goes:
Strong points: the eyes are highlighted and, still, in harmony with the others aspects of the face; maybe the angle of the eyes, especially his right one, could be more accurate (more vertical and with a better 3D effect), but still, amazing job. I loved the lifted eyebrow and the contracted mouth, constructing together the typical facial expression of the caracter. The nose is perfect. The light/dark effect is a very strong point in your draw. The forehead is also very good, and enhances his surprised/ scared/ ironic expression.
What could be much better: the ears, hair and neck, witch seem somehow unfinished, quite simple and differing a great amount from the rest of the drawing.