Chinchilla can't absorb food

I have a 14 year old male chinchilla named Smokey who cannot gain weight. He still gets hungry and eats, but it just goes straight through him. He has gotten very weak and stopped drinking from his bottle. I am feeding him Critical Care with Pedialyte in addition to his regular food and hay, but he is starving. He has lost all of his body fat and I can feel all of his bones. He lived with two other male chins that are fine. I separated them and put him in a guinea pig cage since he is too weak to jump or walk up ramps. I was just wondering what this could be and if anyone else has ever dealt with this problem.