If this sounds like a lecture, it is not meant to be ... just help and explanation of possibilities ... normally I wait for an answer to the questions, but this sounds much to serious to wait, so I am listing questions and possible answers side by side ... for the sake of time and possible seriousness of the problem.
How are his poops? Wet, just moist, or dry?
Wet and mushy/sticking to everything is a form of diarrhea. Give burnt toast and ask the vet to check for Girdia.
Moist but firm is normal.
Dry, small and hard is a sign of dehydration and/or the digestive system shutting down ... not good at all.
Little poops that are hard and dry and very few in number, or no poops, is a good possibility that the chin's bowels are compacted or constipated ... this, for a chinchilla is very serious. Normally, I do not suggest nuts of any kind ... but when this happens one or two almonds seem to help, if you get to it in time. When a chinchilla's digestive system shuts down, he can die within a few hours.
Are you able to see the outline of his spine? This is a sign of dehydration.
Is there a loss of weight? If so, this can also be a sign of dehydration or other illnesses.
If he is not eating, let's tempt him with a raisin or two ... his favorite treats ... anything to keep his system going.
Pydealite instead of water. They usually like the apple flavored one put out by Gerber Baby Foods. Give it to him via an eye dropper if you have to. But he probably is in need of liquids. Even sugar water might tempt him at this point.
Can you post a picture of him?
This may sound stupid, but I've seen it happen to many times in the past and I do not know your experience with chinchillas ... so I will ask ... are you positive this chin is a male and not a pregnant female in labor?
Jo Ann
P.S. Please keep us posted.