Young rays have small mouths, and I have SELDOM had a ray under 6-7" eat shrimp for me. I don't think they can chew on it.
Did you read what I said earlier? Try *CHOPPED EARTHWORMS* 1/2in. in size. or LIVE BLACKWORMS.
Almost all of my young rays wil eat this the first day they are imported from the wild.
Once you start getting him to eat live foods well, he should recognize humans = food, and start to eat whatever you drop at him. I have just noticed, especially depending on the TYPE OF SHRIMP, that young rays can not/will not fit them in their mouth. You cuold try other foods such as squid/fish fillets, etc, but the problem I think lies within the 'consistancy' of the food.. If it is too large and bulky, especially if it is NOT SOFT (like most salad shrimp, it is very hard and chewy), then a very young ray will have a hard time eating it.
For Example: Recently got a batch of 4 Motoros in.. 2 of them about 10", the other 2 at about 8".. The 2 larger ones immediatley ate prepared shrimp, the 2 smaller ones would only eat chopped worms, and refused shrimp.
It's like your trying to feed an infant steak because you wan't it to eat and grow fast, start off with baby formula!