Sorry about your star. If you haven't added any new fish as trigger, angel or large wrasse then I'd say he might be starving. They eat alot of critters out of the sand and can deplete a sandbed quickly. I could be wrong and hope I am. As an alternative you might try feeding a small portion of popcorn shrimp. Purchase uncooked and raw shrimp. I'm not sure if he will eat it or not, but it is worth a try. I would lay the food near a good leg and see if the smell of the shrimp entices a feeding response. Watch to see if it eats. I use to hand feed this to my green brittle star to keep him happy. I also do this with my red serpent star every couple days.
I don't know if this will work, but it is worth a try.
Good luck!