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- The Chiarian
So sorry no one has replied to you sooner. You said Pekin duck and referred it as a "he" so I'm assuming this is a Pekin drake. Is there any way you can take him to a vet to be certain if it's gape and not a respiratory infection?
If you suspect gapeworm then the most effective wormer I am aware of for gapes is one with the active ingredient fenbendazole (Safeguard equine paste or Safeguard liquid for goats). If you decide to try that then you can ask here about effective dosing amounts depending on what you get. You will need to get a weight on your drake.
In the meantime, make sure (if you haven't already) you've isolated him from any flock members and keep him warm (about 85 degree).
I'm not an expert and just wanted to reply with some suggestions until a more experienced forum member can give you additional advice.