I think he's right that we do need to begin colonizing another planet relatively soon, why keep all of our eggs in one basket, but I'm not sure his 100 years is based on anything. In November he said 1,000 years, now he's saying 100. Maybe he didn't get the kind of reaction he wanted and that 1,000 years might as well be 100,000 years to most people (it's hard to wrap our heads around a thousand years), a hundred years is much more "real" and emphasis we may not have much time.
As far as I can tell it's not based on any real numbers, we just have a lot of growing problems (population, climate change, etc.).