Mocha had that problem a couple of years ago.
Temporary Fix: Use oatmeal shampoo and conditioner and not bath so often (I bath him once a week. During the dry skin month, I only bath him once every 2 weeks).
Long term fix: Add more fish to his diet. I mixed the Orijen Six Fish Formula to his regular food and the dandruff went away after a few weeks. So I kept that in his diet from then on. No more problems what so ever, even after I went back to bathing once a week (normal shampoo, not oatmeal)
Six Fish Dog | Orijen
I use Orijen because I don't like supplements. However, since Orijen is not easy to find locally, I heard you can buy fish oil and add to the food. You might want to give that a try.