Even with shedding the shell should not be soft. The addition of the UVB light should help. Even if you can get him outside for 10-20 minutes a couple of times a week that would give him a big boost.

Wax worms should be considered a treat as they have little nutritional value. Try some small crickets, mealworms, feeder guppies. Most people give their turtles Reptomin as far as commercial food goes. What's the water temperature?