Too Much Food?
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Is making hacking sounds while shifting his neck and throat repeatedly, swallowing? If so, he's just got food caught in his throat. Our ducks did the same. They sometimes wanted to swallow their food without water and then it got stuck in their throat and they walked around for 10 minutes just hacking before they finally smartened up and drank some water.
If the duck's mouth is open and is breathing hard, it is panting because it is warm (looks alarming the first time you see it).
What kind of food do you have? Mash? Crumble? Pellets? Pellets is best for ducks once they can eat them, because they can swallow it easier. We gave our ducks crumble before and it would get wet and get stuck in their throats and they'd choke on it a little bit. If you can find starter pellets - usually they're made smaller so they can be swallowed by baby birds - they could help if you aren't using it already. If he's eating, drinking, pooping, and acting otherwise normal he should be fine.