Sorry, I waited too long to edit!
I can see the pictures, and there is definitely something odd going on with that tooth. I'd go to the vet and have the back teeth looked over to be certain that they haven't overgrown. The front teeth look like they are a little long. The normal bottom tooth have worn that strange pattern into teeth, they should be more straight across on the top. I'd probably have the teeth buzzed down to align them better and see if that tooth is still growing in normally.
That tooth could most definitely have been injured in a fall or getting caught on the cage, like Peggy said. A vet visit would help you figure out what is going on in his mouth!