I have the same problem- a cat with severe dandruff problems. I know that the problem is directly related to his diet, because when I feed him richer and fattier dry food, the dandruff goes almost entirely away. Unfortunately, my cat is a senior cat (9 yrs) and he also gains a good deal of weight when I put him on this type of diet. To compound the problem, he never washes himself, so although I brush him, that's the only time his skin ever gets cleaned.
I have another cat that is very fastidious, and she never has this problem, no matter what food I give them. Is there any way I can continue with the reduced-fat diet and yet also give him something that will eliminate the dandruff? Maybe a vitamin, or a special oil that won't jeopardize his health by putting so much weight on him?
He has had this problem consistently for about 5 years now.