Good morning, I'm Dr. Sara. I'm a licensed small animal veterinarian who works exclusively with cats and dogs. I'm really sorry to hear that you've been having this difficulty with Ravi. The "mystery anorexia cat" is one of the most frustrating conditions I've ever experienced as well.
I want to caution you that if you've already worked him up, especially with a board certified internal medicine specialist, and spent that amount of money, there may not be all that much more I can add in this venue, but I'll do my best.
Truly, if I had a patient (and I've had a few like this) who are looking entirely normal and healthy but just refuse to eat, I put in a feeding tube. An esophagostomy tube is very simple to place and use and can be used to feed a liquid diet for as long as needed. I would rank pancreatitis high on my list of potential causes, and sometimes the only way to diagnose this for sure is a pancreatic biopsy, although there are some blood tests that can give you an idea (a spec fPL). I would also consider hepatic lipidosis very high on my list of potential causes and again it can sometimes only be diagnosed by liver biopsies. Sometimes I can diagnose it with noninvasive ultrasound guided liver aspirates under sedation, and many times we diagnose it "presumptively" by saying "if it looks like a duck, swims like a duck, quacks like a duck, then it's probably a duck". Treatment for pancreatitis is typically a combination of pain control, anti-nausea medications, and time. Treatment for hepatic lipidosis is simple - they have to get nutrition. Routine feedings through an E-tube can cure hepatic lipidosis. I'd also consider inflammatory bowel disease for any cat with GI signs that don't respond to symptomatic treatment. Another disease that can cause vague symptoms and doesn't show up on blood work, unfortunately, is cancer. There can be infiltrative cancer throughout the GI tract and once again, it may not be evident without a biopsy of the affected area.
If nothing else, the placement of an E-tube would allow you more time to sort out what the primary problem is. I suppose it's possible that it's now a learned behavioral aversion - he has just had so many bad experiences with food that he's no longer interested. This is especially true if he's been force fed quite a lot - cats usually hate that sort of handling.
It's also certainly not wrong to talk about doing endoscopy for GI biopsies or an open abdominal exploratory to retrieve liver, pancreas, and GI biopsies. That's the next diagnostic step, anyway. As a last thing to mention, if biopsies aren't going to happen, then a trial of steroids can be helpful. Prednisone can help tremendously for both inflammatory bowel disease and GI lymphoma. The difference is that prednisone can keep IBD in remission for a very long time, whereas pred only really helps lymphoma patients for a few months, tops.
Please let me know what other questions I can answer for you!