My Cashie gets a bit of dandruff sometimes, he's a black cat so is quite noticeable. I fed him Royal Canin Skin & Hair kibble and added a wee bit of salmon & flaxseed oils to them as well, just rubbed it on my hands and then ran them through the kibble. It worked, within a few weeks his coat was flakeless, and later on his hair grew longer, shinier and really luxuriant.
I think his started as a result of a bit of an allergy to fleas so the diet was only part of the cure, I waged a many pronged attack on the fleas as well. It happened again this Spring though, he rubs his little ears bald but he doesn't have fleas or mites, maybe its a pollen allergy?
Cashie loves his brushies though and that certainly seems to help the skin & coat if your cat enjoys that, although I know some just hate it.
Anyway, try tweaking his diet to include some omega oils (just a teensy amount) and it may help it and I don't think they do any harm anyway, we all need our omegas but obviously as already suggested, mention it to the vet if you are worried.
(I don't know why I don't try the oils myself actually as the result on him was so spectacular.)
