You sure? In the long term, like that an academic like Hawking probably had in mind, population definitely has been increasing exponentially: http://en.wikipedia.org/wiki/Population_growth first chart.

The population is "set to level off"? How are you certain Or not:

http://en.wikipedia.org/wiki/World_population first chart.

Estimates vary. But you can't be any more certain that our numbers will start to decline any moreso than Hawking is that they we'll continue to multiply.