Looks pretty nice, maybe just work on your lighting techniques (thesparko went into detail on that a little more below) and try to change the hue as well as saturation and lightness when you choose your shade and highlight colors.

Overall nice work :)

Could you try and make the light source come from a top left/right corner?
doing it in the middle is oddly unrealistic, as the light source (sun) is usually coming from the sky!

Also, for this type of resolution, no two pixels next to eachother should be the same shade exactly,
this is a good example of what I mean: www.planetminecraft.com/skin/g-hikaru/

great skin though!

LMAO, sorry. But the shading in the stomach area makes him look fat, nice job though.

I like it! But maybe work on the colors :)