If your snake has access to a lot of climbing/exploring areas in the cage and eats good rodents, then the problem should not arrise. Snakes are lethargic in the wild, too (when not hunting) and so a bit of activity is enough to keep their muscles toned. Is he obese, or simply heavy?
How long is he and what's he eating? What size cage is he in?
You might want to slow down feeding a bit and ad some furniture for him, or let him outside to roam around (supervised of course) every week?
Curious to hear everyone else's take on this...