i'll be somewhat more constructive than the others... i think you need more and darker shading on his skin, and his clothes need folds to look natural and to give them more depth and realism. his skin and jacket needs more contrast too, i think.
plus, i dont think you should shade the edges only in every field not every line should have a shadow after all. you could add some texture to the background to spice it up a little bit if you feel like it.
dont take it too hard, its difficult to get better if no one tells you what to improve. these are merely my thoughts though. someone else might tell you to work on entirely different areas... ^^