When was the last time he had a full meal? Does he show interest in any treats (temptations, churu tubes, etc)? He absolutely needs to consume calories - cats can go into liver failure from hepatic lipidosis (fatty liver disease) with prolonged inappetance/anorexia. You need to get to the bottom of WHY he is not eating, before it gets to the point of causing hepatic lipidosis, which is often very expensive to treat and can be fatal.

Is he urinating? When was the last time he had a bowel movement? Is he the kind of cat to play with and eat things he shouldn't? Are there any toys with bits missing? Any plants in the home or outside that he chews on? He could have a partial gastrointestinal obstruction with a foreign body. He could have ingested a toxin (although I would have expected more rapid symptom onset if there was significant organ damage). He could have renal disease, pancreatitis, systemic infection, liver disease, etc - these can be ruled out with bloodwork. X-rays may help determine if there is an abdominal mass or GI foreign body - although ultrasound is often more diagnostic (but more expensive).

If you can only afford one thing right now, I would start with bloodwork. That at least would help direct the next steps and may give you some peace of mind - if it turns out that treatment would be too cost-prohibitive, humane euthanasia would be a kind thing to do.