What are you feeding Chopper? A lot of skin issues have been food related. If you are feeding a chicken based food, duck may be a bit close in protein to the chicken, so you may want to try either a Lamb & Rice or perhaps the Venison or Fish....
California Naturals makes an excellent Lamb & Rice--I have used that with Buster and his coat was amazing on that. I also used the Natural Balance Duck & Potato and liked it for the most part, just had an issue with looser stools. Solid Gold also makes a great natural lamb kibble. Just keep in mind that if you do switch foods, watch the treats also.Many contain the same food ingredients. So, if you are eliminating chicken first, make sure no treats you give him have chicken in them.
Just some suggestions on what to try. Good Luck and keep us updated on Chopper's progress.