Then it could be a birth defect, if he hatched with it- however, it could also be an early case of MBD.
Do you have a UV light for his tank? A proper, long tube one, not a coil, and what brand? Do you dust his crickets and greens with calcium powder?
If it's not something he hatched with, or an injury, then it's MBD from maybe an incompetent breeder, I can't know for sure, and as long as you are providing him with proper UV and calcium, it shouldn't get any worse.