Lots of times other diseases kick in and take over once they're weakened by the 1st disease. E.coli is one of the worst offenders. All birds have e.coli in their systems. It's just sitting there waiting for some other disease to wear them down so it can slowly kill them. I believe the kindest thing is to let him go. I always feel like I've let them down when I don't have the guts to end their suffering.