Use the bird
A big bird like a duck really lays a good scent trail. Use a bucket with water in it to soak the bird and leave a nice strong scent trail for the dog. Re-wet the bird while laying the trail if you have to; a strong track will help keep the dog working and encourage him to trust his nose and raise his confidence level. Likewise, pull a breast feathers out to mark the beginning of the track and leave a few feathers en route to the final resting spot to keep the dog confident and working.
When pup finds the bird, lavish him with praise so that he knows you're pleased. When he returns to you with it, allow him to hold the bird for a little bit while you pet and praise him some more. It will help cement the good work in his head and create a positive association of working through a trail with not only a bird, which is the ultimate prize for a well-bred bird dog, but also praise from you, which is almost as important. That will make the work fun for the dog and it will be something he will want to repeat, and he'll do it enthusiastically.