Usually for a ray to be that thin it is not an all of a sudden thing. I still think I would treat with Prazi Pro if you can get your hands on some. Did you say you got him some night crawlers yet? Get some of them and cut them up pretty small, smaller than the ghost shrimp, you can also soak stuff in with the black worms and some times that will prompt them to eat something they normally wouldn't since it will then smell like the black worms. If he is only eating ghost shrimp and won't eat anything else since he is so thin I would let him eat as many of them as he wants 3 isn't even a tiny snack for a motoro. I have seen my retics on the RARE occasion that I let them play with ghost shrimp clear out 50 of them in a night between 2 retics around 8 inches. I hope this guy pulls out for you. Is there anything else in his tank with him?