I'd say the overall problem is stiffness and lack of volume. The general advice would be to draw more curves; in specific terms:
1) Make the hair fuller and avoid "sawtooth" hairline.
2) Curve the shoulders.
3) Thin the neck.
4) Use curved lines of action to add interest to his pose.
5) Curve the edges of shirt, sleeves, pants to give a sense of volume.
6) Place the eyes, nose, and mouth solidly on the perspective lines.
7) Place the shirt logo in perspective.
The trick is to start with basic volumetric shapes, such as a ball or pear shape, then build on top of that. For more thorough examples, check out the Preston Blair books.
Keep in mind that this is just my interpretation. You can apply the same ideas and come up with something uniquely yours.
Hope that helps.