ok, with all the information you gave, sounds like its just stressed still from being in a new tank.
Id feed him whatever he will eat and give it some time. He just needs time to settle in.
If he was on sand and in with other rays, now being placed in a tank that is totally different, its just weird to him. He is just prob getting use to the new area.
Blackworms will help, but like some others have said, it may be harder to get him off later. Id keep doing what you are doing.
In time, he will settle in and start eating more.
Keep us informed.