First off, gotta say this is absolutely brilliant. The face and the hand are almost photorealistic. The only things that I think would improve it are putting a little bit move shape back into the arm (if you look at the photo its a little thicker and more curved) and making the shoulder on the right a bit move curved too. Very nice though.
I was going to say there is something about the spine. It seems to be missing a bit of harmony to it compared to how a lot of the rest of the figure is represented. Good effort though, really nice.
I think at this point the thing to do is to go through some anatomy books and look up just how everything works together. You've got the shading and stuff in more or less the right place, but it doesn't really have "form" to it, outside of largely the face and hands. His left deltoid is a good example; it's the right shape and has the right tones to it, but it doesn't look round, and it just kind of blends into the bicep more than it should. I think if you studied a bit of what you can't see on the body (muscles, skeletons, etc.), then carried that knowledge as you draw from real reference it should help give form to what you're drawing.