Any pooping? Try milk instead of water so it has some nutrition for now if it's able to drink.
Sadly in nature things can seem very cruel. If this chick has some reason it's not able to eat anymore it's mom would want to run it off because as it gets weak it will attract predators, which will put her and the other chick at risk. So this could be the reason it's with you now. However that's not to say that it might not recover with good care, just that it might not be what you have offered that is the reason it's not eating.
Check it too for lice and mites both for it's health and to reduce the risk to your own chickens.
Then, lets see.... I'd be looking at the crop to see if it's full, or boggy or empty. Full means he is eating, or it's not emptying, boggy is infection with thrush most likely, empty is not eating, so the problem could be higher up like the mouth, or just no appetite. Generally look him over for weight and condition, symptoms of infection or old injury.