a few crits:
cut down on the pillowshading. Set your heart on a constant light source. Right now one would surmise that the light source is directly about him, but in a good few areas the shading seems iffy.
AAAH! Saturation! Seriously, that green is blinding me. Not to mention that the extreme contrast between that green and the darker one is very unsightly. You could easily do without the brightest green altogether, and then just slightly increase the contrast between the other greens (right now the middle green just makes it look blurry).
Fix those and it will look a LOT better. Trust me, right now the shading and the bright green (mostly just the bright green) is really ruining an otherwise good piece.