dogfan, what are you feeding your dog. Dandruff can be a sign of an allergy or extremely dry skin. I suggest putting your dog on a high quality good with little or no grain, and no by-products, corn, wheat, or soy.
Also I'd use a fish oil supplement...that should help some with the dandruff.
Also make sure you aren't bathing him frequently. Bathing strips off all the oils that they need that for their skin and keeps their skin healthy