I've read on here, that sometimes part of their tail will die after some sort of injury. Like getting it closed in the enclosure door, or nipped by a cage mate when they're housed together as babies or at the pet store. I haven't personally had to deal with this type of injury, yet. So I'm just repeating what I've read here.
Sometimes, it will fall off on it's own, and sometimes the area of dead tissue keeps spreading. When the dead tissue keeps spreading, a vet will remove the necrotic tissue. I'm not sure if there's anything you can do at home, until you take him in to see the vet. Hopefully someone that's dealt with this will see your thread and give you more info.