Chinchilla Weight Loss?

I have a 4 year old male chinchilla. For the past few days he's lost A LOT of weight and i can feel all his bones! He won't take any treats and I haven't seen him eat any of his food. His nails are sharper than usual and he's had a couple of whiskers fall out. He's acting lethargic and I checked his teeth for chips but they seem fine. The fur on his bottom is yellow and he's been acting terrified of me. He has no eye or nasal discharge. All he does is sit in his little house all day and night. He's also been rubbing his abdomen on the floor. But i'm mainly worried about his extreme weight loss and major lack of energy. And my parents won't let me take him to the vet because "it costs money". I don't know what to do!!!