I would encourage you to see a vet if for nothing else than a general wellness check and either a fecal float (if he does poop) or the vet can do a vent wash and check for internal parasites. With a snake that's been so badly abused/neglected it's good to have a vet go over them and see what's really up. It may be that the snake isn't pooping because it's so starved that it's absorbing all it can from it's food. It could also be still dehydrated and every creature needs water intake in order to produce feces properly. It really could be a lot of things and none of them can be figured out online really. Best to see your vet when it comes to any creature that's had a long history of such poor treatment.