I'd definately try bloodworm. One of my map turtles loves chasing them and eating them - he doesn't care or perhaps notice that they're freeze-dried when the water current moves them. You can often buy bags of live bloodworm for about £0.50 from LFS's.
You could try hatchling turtle food which is much the same stuff, but in smaller pieces. Try offering some vegetables (shredded carrot, finely chopped lettuce leaves - NOT iceberg lettuce though) or diced apple.
The UVB light when absorbed by the chelonian is used to break down calcium and synthesise vitamin D3. It prevents metabolic bone disease (bones bend and become weak). UV light apparently also helps with increase their appetite.
If they're kept in an aquarium with fluorescent bulb fittings then you can buy a reptisun or reptiglo (by Zoo-med and Hagen) tube to fit into the existing fitting. If not you can buy a UVB bulb that looks like an energy saving bub to fit a standard light fitting. You also need and incandescent (normal or reflector) bulb to provide heat for them.
I suppose stress could be the problem. Is he being bullied? Have you examined his shell for fungus or shell-rot? Apparently RI (respiratory infection) can cause turtles and terrapins to stay out of the water. If you're handling them a lot or there's loads of activity near their tank try quieting things down and leaving them alone.
Hope that helps,
Ian.