It might possibly be an opportunistic bacterial infection or a fungal infection, but proper nutrition should give your duck's immune system what it needs to fight it off if it is. I don't think ash would hurt, but i think good nutrition would help more.
I forgot to add to the earlier post...
Id say part of the problem is a vitamin deficiency. As the smallest he probably isn't getting a fair share of the food. Plus, ducks require more niacin than chickens. Liquid B vitamin complex is cheap, readily available, and can be added to a small amount of water-3 or 4 drops to a cup of...