So it sounds like he should be fine if I introduce him to a new duck gradually. It looks like none of you guys live near me, and I prefer not to include my location (even pretty general location) online. Nevertheless, thank you guys anyway, and I'll be sure to check craigslist for ducks closer...
We've had two ducks, a khaki campbell female and a swedish blue male, for about five years, but recently the female went missing and we fear she was carried off by a predator. The male seems pretty lonely, and I think it's especially important that he has some sort of companionship with winter...