Elevated liver enzymes are not always a sign of pri***** *****ver disease and the liver enzymes are often elevated due to other disease processes nearby in the abdomen. It would not be common for liver disease alone to cause that degree of weight loss in a cat that was not acting sicker. In other words, if this amount of weight loss were due to liver disease alone, we would expect him to be sicker as the liver has so many important functions. Weight loss is a very common scenario in cats and it is likely that there is disease present, but that it is hiding very well. Most of the time with the symptoms you describe, the most likely location of the problem is the GI tract. Some cats have a low grade flare up of pancreatitis which can be a challenge to confirm as it is not always evident on routine bloodwork. Along with that, many of these same cats also have a degree of inflammatory bowel disease (IBD) or even slow growing intestinal cancer. Unfortunately, either of these can be present and there can be no other abnormalities noted on bloodwork, xray or even ultrasound, although these tests should be run to rule out other conditions that present with the same symptoms. There is a specific GI blood panel that can be requested and if abnormalities noted, can lend support to either pancreatic or intestinal disease. The only way however, to confirm the diagnosis is with a biopsy of the intestine. This can be done via endoscope, but is better performed with an exploratory surgery. The problem with this is that many cat owners, like yourself are not wanting to pursue the diagnosis by these means. It is for that reason that many cats are "suspected" to have IBD or GI cancer as the cause of their weight loss and anorexia once all of the other easy to find things have been ruled out. There is a syndrome known as triaditis where the liver pancreas and intestine are all to some degree involved in an inflammatory process. He may be experiencing this and his liver may or may not be involved, but I would be willing to bet that there is some degree of intestinal involvement.
Even without any further diagnostics, there may be ways to help him continue and even increase his quality of life with medical management. He may benefit from a course of corticosteroids. There are other medications and supplements used to manage IBD as well including appetite stimulants and anti-nausea meds. Steroids help many cats with either of the two intestinal conditions to regain their appetite and gain weight, but if there is intestinal cancer, the result may be short lived. many of these cats also benefit from Vitamin B12 injections as cats with intestinal disease usually are low in B12.
In answer to your question about when to consider euthanasia, using total refusal of food for days is a fair estimate, but many cats with intestinal disease have poor appetites as part of the disease and that is where the steroids and appetite stimulants play a large role. They don't work in every cat, but they help enough of them to eat better and even gain weight that it is alway worth a try when we are treating empirically for a suspected disease. Most of these cats aren't suffering, they are just losing weight.If you can jump start his appetite and at least maintain his current weight, he may continue to enjoy life for quite awhile.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.