I'd say he's probably just really thin. Maybe like I was when I was younger? I was never really underweight or malnutritioned or anything, but I was still thin enough that you might think I was if you saw me. Looking ill when you're not is pretty friggin' awkward, I seem to recall a teacher thinking I was anorexic or something. Which they brought up in front of my classmates because apparently they were an idiot with zero tact.