It's an uncommon kink related to masochism. It is dangerous but can be done fairly safely. The penis is tougher than it looks. It can handle a lot of abuse without immediate injury. Your teeth however can sheer the skin off someone's arm with minimal effort and can easily tear a penis from the torso if enough force is applied. Many men into CBT experience ED at a younger age. Theoretically the repetitive intense pain damages their nervous tissue.
Bite but don't grind or pull. Pressure is much less dangerous than torsion, less likely to damage the ligaments of the penis, less likely to break the skin cause actual injury.
It is possible that he has a sexual need for pain but just no experience with other avenues of extreme sensation. See if he'll experiment with safer forms of pain or things that wig you out less. The buttocks and back can take some surreal punishment without serious injury, it's the same endorphin high. Clothes pins can cause much more consistent and measurable pain than your teeth with a safter wider surface area. And of course never take things outside of your comfort zone just to please him.
Good luck!