Is he the only one that you have? I'm new to ducks too, but everyone has told me to have more than one because they get lonely. I do know that ours preen constantly!!! But no bald spots....maybe it's where the down is going away to make room for feathers. I have a chick that went almost bald...
Ducks need water that they can put their entire head into because that's how they clean their sinuses out. Try putting water in the sink or a large bowl and let him have at it! Also, makes sure it has water at all times because that's how they wash down their food.