When I saw him in the second book the word that came to mind was 'thicker'. Like he's been spending more time in a computer lab/art lab instead of mandatory P.E. classes. It does not take much time for that to add up and it's been indicated (or at least that's how *I* took it) that there has been a few weeks between the first book and the second. Cheap food/junk food + classes and labs = your body is gonna change.

Plus that time he was posing in the mirror before his date was cute and a good reminder that guys are just as sensitive to their body image as girls. Equal opportunity all around!