tee tree oil. 🙂 The same suggestions that everyone has been using for head lice also works for dandruff. It heals the scalp too where he has undoubtably been scratching.
Use Donna’s suggestion for it… Buy a bottle of baby shampoo… The tearless kind. Pour out 2 ounces and then pour a two ounce bottle of tea tree oil into the bottle of baby shampoo. (she suggested lavendar, but sub the tea tree instead.) shake well and use that to wash his hair… Lather, leave for about 5 minutes, rinse.