Sad news....

[ReptilesK2]

The problem is you used mealworms. Waxworms are better for bulking up, but mealworms MUST be fed prekilled. Their exoskeletons are too tough to be broken down by digestive enzymes, meaning your dragon was likely eaten from the inside out. Check for any small holes in his body, I'm sure you'll find some if you fed live mealworms.

-edit- Another killer may have been the lack of a UVA/UVB. Since dragons aren't nocturnal, they need proper lighting to survive.

Sorry, but this is false information (mealworms eating their way out).
It is not true at all.