I can't imagine what could be wrong with him if there are no signs of injury and he doesn't seem to be in pain. Very odd. I too would be afraid of some illness so it's good they are keeping him isolated.
Is it possible that the neighbors hurt him? Is he so afraid of people that he could be scared to move? Is he sitting up when he eats?
Sorry for so many questions but it is really strange. I will pray for the little guy and I hope he is ok. Would the shelter where you volunteered be able to help at all with a vet?