Oh dear, poor duck.
I think that just using the neosporin is all you'd need. Next time you dress his bill just use that. Other than that Betadine would also be good as an alternative.
Duck bills are jam packed with nerves. So this injury will be extremely painful - like as if you had skinned your tongue. For that reason I'd keep him quiet and in a dark warm place, to help him get over the shock. Can you keep him inside the house? Keep his bedding clean. Keep his water clean and keep him away from other ducks for now.
How did the injury happen?