It is incredibly heartbreaking to go through loss like that.
I can't recommend getting a necropsy enough, to be bold here, you absolutely must do a necropsy to verify what is happening to your flock. If it is Mareks, then read up on that, only get vaccinated birds, keep everything as clean as possible and you can likely continue to have a healthy flock to enjoy. Just be aware of how you can spread Mareks via your clothing and shoes as you go to feed stores etc. If it isn't Mareks, the necropsy will tell you what it is, and same, whatever the diagnosis is, research, treat / prepare / plan, knowing what is happening will help you know the path you need to take.
As for how he has hung on longer than the others, it is likely either genetics or he has a strong immune system.
Please keep us posted, I'll follow this thread and be sending you support from afar.
A few questions, how old were the chicks you got from the farmer, were they incubator hatched or broody raised? How long ago was this, and how long after getting the chicks did you start to have issues in the flock?