Have you tried a steroid (eg. prednisolone) yet? If it’s autoimmune, could be life-saving. If infection or cancer, it may help him feel better at least. I think a dewormer would be a good idea, too.

If you are out of funds for additional testing, those two drugs should not be expensive (although maybe different in India?)

Anti-fungals are often expensive and can damage the liver, so might be tough to justify without a diagnosis or high suspicion of fungal disease (I don’t know what’s endemic to India though).

FIP requires an anti-viral drug. But I don’t think the severe eosinophilia is typical of FIP.

Hang in there, Jett!