Has the vet ruled out gum disease? Is the cat drooling?
Was kitty obese prior to dropping all that weight? Have they ruled out a throat obstruction? Is the cat sneezing or congested? Because if a cat can't smell, he won't eat.
You can try a few things. Add just a tiny bit of garlic to the wet food and heat the food in the microwave (cover it with a damp paper towel first) for about 30 seconds.
If the cat is a stray, you can add a very small amount of organic catnip to his food. But if he is a domesticated breed, do not do this!
Try boiled chicken moist with broth, shredded cheese, cottage cheese, plain yogurt.
If he was initially plump before he lost this weight, you want to talk to your vet about Hepatic Lipidosis. It basically is when the cat has lost so much weight, that internally the cat's body forces the cat to metabolize it's own body fat rather than the energy from the food (since there is no food to speak of)and this just compounds the problem because the body thinks it is full and the cat will continue to not eat- then the liver becomes invaded by these fat cells and it is downhill all the way from there.
At this point, try anything you can think of to get this cat to eat.