I would go with a grower feed, like Blue Seal Grower Cal. I think the duck has been missing many vitamins. You can get (you may already have) poultry vitamins.
Some ducks are more sensitive to nutritional deficiencies. So it's safe to go with a grower ration that has all the nutrients.
Sorry your duck is ill-you cannot feed ducks cracked corm as their feed-they need alot more than that-all my ducks are on grower and layer pellets ( because they eat all the chicken food) lots of greens!!