Thanks for the additional information - I know you have already spent a lot of money trying to figure out what is causing this weight loss in your kitty, but since no answers have been uncovered so far, it is likely that you are going to have to do some more testing. It is only when an accurate diagnosis has been reached that you are going to be able to know what treatment is needed.
First, as far as blood work goes, I have to wonder if there were some abnormalities in the liver enzymes because of the Denosyl medication - this is a medication that we usually gives animals when they have some type of disease process going on in the liver and is not usually used for IBS alone. You might want to ask your vet again about the lab work that was done. Also, since this has been going on for some time, sometimes rechecking blood work will reveal abnormalities that were not present the first time it was done.
I'm glad that they took x-rays and that they were normal, but there are a lot of diseases that won't show up on an x-ray but will show up with an ultrasound. Also, if any abnormalities are seen on an ultrasound, they can often be biopsied and a specific diagnosis can be reached that way. Most clinics do not have ultrasound, and this often requires a referral to have this procedure done.
The only way to diagnose IBS and several other conditions that cause vomiting and diarrhea is with biopsy of the gastrointestinal tract. This usually can be done with endoscopy, so while it is still requires anesthesia, it is not a major surgery. It is likely that the only way to find out what is causing your kitty to lose weight is to have this test done.
Right now, it is really important that we get your kitty eating again - if that means giving him human food, so be it. Try to stick to meat or poultry as much as possible and make sure that it is low in fat. Cats that go without eating for very long will get a liver condition called hepatic lipidosis. It is possible that this is going on already in your kitty. This is a serious disease and usually requires aggressive treatment such as feeding tubes to get them through.
Unfortunately, cats that suddenly want to eat weird things sometimes have a condition called lymphoma, which is a cancer originating from the lymph system. In cats, it can affect their GI tract. Obviously, this is very serious disease, so the sooner it is diagnosed the better off we will be.
Again, I know you have already spent a lot of money, but I really think it is time to seek out an internal medicine specialist so that you can have the more advanced testing done that I mentioned above (ultrasound and endoscopy) as that is the only way you are going to get a diagnosis here. Most vets are very happy to make arrangements for a referral to a specialist, so please don't be afraid to ask your vet for this. I know it will be expensive, but it really is the best way to help your kitty.
I hope this gives you some ideas of where to go from here. Please let me know if you need more information.