http://www.thepoultrysite.com/...
There's a lot of things it could be, and none of them are good news. It doesn't seem like it's a good idea to keep him around to suffer and die, and perhaps spread what he has to some other birds. The consequences of keeping him around to die far outnumbers the small amount of discomfort of euthanizing him. IMHO
The above link lists many of the possible diseases.
