Herbs for Helping a Thin Cat Gain Weight?

I have a cat that is just too thin – he almost looks skin and bones. He is an indoor-only cat, with all his shots. Our vet can't find anything wrong with him, other than a stuffy nose. It seems he just can't smell his food very well, so he doesn't eat much. We put him on medicine - clavamox - but in the meantime, he is still losing weight. Are there any herbs that can help with the absorbtion of what he does eat? Also, what herbs can we give him to promote his will to eat? We even tried giving him warm cooked chicken meat, something he could smell better, but even that didn't tempt him much.

Why was Clavamox prescribed? Did the veterinarian feel an infection was brewing somewhere based on bloodwork or exam? What have you tried to feed him thus far? Have you looked at the diets recommended by Richard Pitcairn, "Natural Health for Dogs and Cats", "The Complete Herbal Handbook for the Dog and Cat" by Juliette de Bairacli Levy or "The Barf Diet" by Ian Billinghurst? You might find this information useful. As for the stuffy nose, perhaps clumping (and dusty!) cat litter could be implicated or an environmental allergy (carpet, paint, household cleaners, detergents, perfumes, bodycare products, etc.). Cats have much more sensitive noses than humans; you'd be amazed at some of the things they find problematic which we completely ignore!
How are his bowel movements? The colour, texture and look of stool can tell you alot about the efficiency of his digestive tract. Is he vomiting? Has there been any diarrhoea or constipation? Blood or mucous in his stools? How are his teeth? How about stress levels? Anything change in the house over the past few months? Loss (or gain) of a littermate? Moved to a new location? New (or loss of) roomates/partner? Any stress in your life? Cats are often the mirrors of everyone else in the home. Any recent operations? These are just the beginning of possible questions. As you can see, eating difficulties can stem from any number of sources. Recommended herbs would depend on the cause.