Post a pic & we could probably give you a better response. if it's just dirt, you may be able to bring him in and help him wash it off.
I have 1 muscovy duckling that almost refuses to take a good bath, she wont wash her tummy so its always dirty & i have to help her sometimes....she also happens to be white
Have you tried to let him bath in warm water? That may just do the trick.
After that you could try Dawn dish washing liquid with a sponge. See about it at: http://www.dawn-dish.com/en_US/savingwildlife/specialedition.do
You can also try a mild baby shampoo. A soft toothbrush may be helpful.
Katharina