There's not too much wrong with the shading, except perhaps for the fact that it isn't really indicative of his form because it is lacking contrast. You really need some darker tones in there. Also, the anatomy looks decent, but could certainly improve. If you drew from reference, then you should have drawn what you saw instead of what you thought you saw. Case in point: the abdominal muscles. If you didn't use any reference, then I think it's time you start regularly doing that. Life studies are the best, but not everyone has convenient access to nude models, so photographic reference comes in at second.
Also, go read some
Andrew Loomis, especially the
Figure Drawing for all its Worth and
Drawing the Head and Hands. Gold mines, I tell you.
Just keep on practicing, and you'll get there.
