The anatomy as a whole could use some work unless you somehow intended for his limbs to be that way. The arms are longer than the legs for intance. I suggest using a reference while doing this kind. Secondly, what is the lightsource? You definitely want to practice with the contrast of your colors. Try reading some of the tutorials on the site. Last but not least is perspective. Im not sure what perspective you're going for with this piece, so some info on that would help.