Not necessarily...if he doesn't appear to be overly thin, than he's probably not underweight. I have a 2009 female that's 625ish grams and a 2010 female that's 1,800 grams. At this time last year, I had a 2008 male that was less than 400 grams. I don't feel that any of those 3 snakes had anything wrong with them...simply put, not all snakes have the same eating habits, metabolisms, or grow at the same rate.
Superpop, if you're not seeing a slight bulge, it may be time to move up in size to weaned rats...or give him an extra mouse.