Could be a number of issues. Is there a large crab or a mantis shrimp that may have injured him?
Stars will heal pretty amazing wounds given good conditions. If you have a refugium, you might consider putting him in there to recover. If a predator took a chunck out of him, he'll need a safe place to recover. He might be better able to avoid the predator when he has all his arms back.
He most likely is a goner IMO. They seem to deplete the sand bed (or there was never unuf to sustain) and begin wasting away, like they're melting. Does the sand stick to his remaining body? If so, that's another sign he's on his way out.
"Life may not be the party we hoped for but while we are here we might as well dance!"