Ball pythons that are gearing up to defecate can look pretty bloated toward the rear--that's fairly normal. If you see that in your snake, you may want to reconsider handling him for a bit, lol.

Dehydration can delay things a bit, yes, but sometimes they just seem to like to store things up for a while. Snakes in shed will often do this, and defecate immediately after shedding (or even during).