He's not in favor of population reduction (or, at least he's never actually said so). The out of context quote is from a Ted Talk he gave talking about reducing carbon emissions. He was talking about slowing population growth.
https://www.ted.com/talks/bill_gates_innovating_to_zero/transcript
If I'm not mistaken, he was actually saying: better vaccines combined with birth control-> lower birth rates in developing countries (because they will have reduced infant mortality). He wasn't super clear on that point:
"First, we've got population. The world today has 6.8 billion people. That's headed up to about nine billion. Now, if we do a really great job on new vaccines, health care, reproductive health services, we could lower that by, perhaps, 10 or 15 percent. But there, we see an increase of about 1.3. "