Two days ago, one of my 4 month old chickens was sitting strangely upright, looking very ill (sitting kind of like a penguin, with his tail on the ground and his back straight up and down and tail dragging in the mud). I discovered that he is very thin and I brought him inside (the nights have been below freezing, in the upper 20s). I've been feeding him as much as he will eat, but after one day inside, he has stopped eating -- he doesn't poop at all, so I think something is going on digestively, but I can't diagnose it. Now it's been 2 days and he seems more and more listless. He halfheartedly pecks at food but doesn't even eat anymore. At first I thought perhaps he was injured but I found no signs of injury and his thin condition (I'm so mad at myself that I didn't notice earlier) tells me it's a different problem. No mites or insects anywhere. He can't walk well. kind of hobbles only when he needs to. He's been sitting in a lump all day and not even touching his food now. He is still not pooping, except for a teeny tiny bit of white once or twice. I'm sure it's a digestive issue, but whether its a blockage or cancer or something like that I can't tell. His crop seemed ok, his belly is not swollen anyplace. I am not sure what to do at this point. I hate hate hate that I don't know what's wrong. I don't think he's going to make it, and I don't want him to suffer, but I'm also not sure if I can still do anything to help. Advice???