I've just come upon this post and was going to suggest Marek's Disease might be the cause when I got to the lab report. Many people do not realise that Marek's can cause sudden death as well as the other more well known neurological symptoms. Quite surprising that his body condition was good. Often you find they are thin. The chances are that he had problems with internal parasites as a result of his immune system being compromised by the virus, but it would be a good idea to have a faecal float test done on your remaining flock to establish their parasitic load with a view to deworming them and perhaps monitor their individual weights or body condition by checking their breast muscle on a weekly basis.... checking them at night whilst they are roosting is the easiest way. Adolescent birds are particularly at risk of Marek's so pay particular attention to any other youngsters you have.