Hm, I dunno what to say about this piece. I like it a lot, but ulitmately it seems pretty simple, and I think you are aware of that. And that's fine, I think that's an aesthetic choice (I really like this sort of minimalism), but I don't have much to say because it seems to me if you wanted to take this further you'd know what to do (at least with this newest version, it's a great improvement).
Perhaps there should be more contrast between the two lighter blue shades. I don't like the hair, it seems a little too domey? I'm not sure how to get across a combed look without making it look like a dome, though. Also, maybe his left leg should be a little darker to put it closer to the plane that his left arm is at. His leg pops out a little too much in comparision.
Looking at it more closely, there's that intermediary shade of grey that's not very noticible that produces a blurry effect that I don't like.