Hello there , i also had a smalll stingray a while ago. At first it was very hard to feed , him he would not eat and i was alway throght food in the tank. I even used to put food in my hand and hold it right by him hoping we would take it. Anyway, my old stingray would eat shrimp. i would fo to the regular fish store and bye some coctail shrimp, frest and cut them un and feed to him . after a while the ray would come right to the top of the tank and feed from mt had.
So ... try that good luck with your new ray .