I'm not thinking it's MBD since nearly all severe cases of it have the lower jaw sticking out like an underbite, and this little guy seems to have an overbite if I'm seeing things correctly? If you're able to, you can maybe nudge his mouth to see if it's firm or the bones have some give. Considering you've been giving him calcium with D3, I highly doubt that's why. Only things that come from mind is a unique developmental issue or maybe he somehow injured his mouth while eating, but only a vet can really determine what's up.