They do need more than insects. People that do feed bugs dust them, but also feed a complete diet like repashy, pangea, etc.
Do his legs look bent?
A couple MBD links that might help. http://www.moonvalleyreptiles.com/cr...ne-disease-mbd
A video on youtube showing a crested with MBD
General Crestie health disorders link
If you are working at a store that is a chain, most of them have "care guides" that the stores must follow for their animals. I believe both petco and petsmart are required to feed their crested prepared diet (not sure on bugs).