I've placed ducklings outside at 2 to 3 weeks and never lost one. I've done this more than once and the most recent time, nights had been between 50 and 60 degrees. They did have a nestbox that they stayed in though, with straw bedding. I think ducklings are able to keep warm pretty quickly.
My muscovy hens will cover their young for the first week or so, and after that they pretty much huddle together for warmth. However, with only one duckling I would provide heat for at least a month if he wants it.