Sometimes rays can be hard to get on other foods but they will always go nuts for ghost shrimp. Slowly keep trying other foods as the rays settles in they most likely will eat other things you offer them. Here is a list of what I have found they like and some of what I feed mine now. If the ray is small you might want to start him off on very small pieces or blackworms. I take shrimp, scallops, or tilapia and cut into very small pieces and use that until they get big enough to take larger pieces. Buy salad shrimp or bay scallops those are the smallest you can get at the stores here. I only buy ghost shrimp as a treat not a main source of food gets costly.
Frozen Scallops
Frozen Shrimp
Blackworms
Frozen Talipia
Mussels
Earth Worms