Getting him to a tub of warm water will help with the smell, as far as poop ducks can have all color and kinds of smelly poop depending on what they ate. watery is normal too. Is he eating or drinking? When was the last time you actually saw him up walking around? Could he have been hurt days...
If you bring him in and he doesn't aoppear to be in shock you could also run a warm bath for him let him wash real good that way he'll clean out any nasties that maybe in the wounds.
Grit is important especially if their isn't any available from the ground. sandy soil small gravel can work if you have it or plain grit that's what I keep out for mine. I'd also check him over from head to toe with all those feathers it's easy to miss wounds.
I'm glad you got the drake or they probably would have killed it. Yes feed it what ever you have but treat the injuries if flies get to any open wounds he is history, why would they leave their dogs where they could get hold of their ducks? Sorry make me mad when I hear of this.
Do you have...