As long as your filters can cope just pound him with bloodworms 5 or 6 times a day.
The ray should look like it has a big rear end.
When viewed from the side the back of the ray will be higher than the middle part when the ray is full.
Ignore people who say bloodworms are not nutritious. I have saved plenty of starved retics feeding only bloodworms.
Apart from skinny it looks healthy for a retic. Good bright colours. Just keep an eye on your water with extra feeding