Dead Drake :(

I found my 10 month old drake Oscar dead in his house yesterday morning, still in his sleeping position. I checked his body but he had no signs of anything wrong with him. His brother Darwin (they have been best friends since hatching) refused to come out from hiding behind a shed so I moved him from behind the house (where they had lived together) to where the rest of my gang live so he can atleast hear my other ducks. I've put him in a shed by himself because I'm worried he may have a disease or something and I don't want him passing it on to anyone else.
Does anyone have any suggestions as to what might of killed him? Or how long I should keep him seperate? I would be very grateful for any suggestions.

  Do you have any suggestions as to how long I should keep him seperated?