MBD stands for Metabolic Bone Disorder is basically a umbrella term to describe an nutritional imbalance. If a lizard has received too little calcium, too much Phosphorus and not enough UV, they can end up with weakened, malformed, curved or swollen bones and joints.
It can also affect the jaw, making it appear swollen and off set.

Get him booked in at the vets as soon as you can.