, he's doomed. the rest of his legs will fall off shortly and he'll turn to mush.
most starfish don't fare well in captivity. even the collection process can kill them. it takes a little while though, so when you get him he may look fine, but he's already on the way out.
as far as knowing when they are dead, i don't think anyone can answer that with 100% accuracy, without doing extensive testing, examining these creatures at different stages of their decline. i don't think there is any articles on this to date.
if i am correct, and the starfish doesn't bounce back and grow a new leg, i'm sorry about your trouble. you might try some of the more hardier starfish, like the serpent stars and brittle stars. they seem to do pretty well, and you can hand feed them meaty foods.