He's either very fat or very bloated from gas or fluid...no way to tell from just an online photo. For a snake that is only a year old or less, you'd have to feed
him a ridiculous amount of food to cause this, since they are normally growing lengthwise at the same time. I wonder if there may be a metabolic problem.
Those creases on his side are typical of snakes that are obese but I'm not convinced it's fat. You say he's "very active"? Is he your first snake? (any others to
compare activity levels to?) I suspect he's in a lot of discomfort...I know I would be...he's not drinking salt-water, is he? (do you use a water softener at your
place?) With snakes, you have to read between the lines...increased activity can mean he's hurting.