Hi Sean,
Overall you have a pretty good character. The pose you chose shows a lot of personality already. However, there a few things you could to do to improve his design. For example you might want to consider unifying his stlye a little more. By this I mean making his body more smooth and reducing the emphasis on realistic anatomy, this would be a better fit with his cartoon-like apperance and complement the style of his clothing. I don't know if it is your intention to have a realistic physique, but if you do, you should look at some real world references to get accurate muscle placement and form. It would be good to add some wrinkles and folds to his shirt as well to complement the more detailed body of the character, if you're going for that realistic-cartoon hybrid look.
Javier