I'm glad he has access to forage, but most likely he isn't getting enough vitamin B. It's a common deficiency in waterfowl. It would still supplement his foraging with a balanced feed. Just an ordinary bag of Purina Flock Raiser would help the ducks health.
Corn is not nutritionally complete. Do you feed him anything else in addition? If not, he is likely suffering a vitamen B deficiency.
Just continuing to think, but he's also probably protein starved. If he isn't on really good forage in addition to the corn, he needs poultry feed and...