My best suggestion to you is to have your cat checked out by a veterinarian. They can examine your cat overall of course, but also examine his/her skin and coat, and do any necessary tests to help with that problem. This will help to determine the cause of the dandruff, as their can be quite a few different ones.
Some do-it-yourself remedies could actually irritate the skin furthermore, potentially causing more problems or worstening the situation.
Hope that helps, and let us know how he/she is progressing!