More people living doesn't necessarily mean people living better lives. In a finite world with limits to growth, relaxing one bottleneck only leads to the next bottleneck.
(I use the term limits to growth in reference to the book Limits To Growth -- http://joshuaspodek.com/the_best_book_on_the_environment_eco... -- which strongly informs my perspective and I recommend to others as particularly relevant here. It offers a systems perspective -- http://joshuaspodek.com/systems-perspective-population -- necessary to create effective strategies to help everyone in the context of a finite planet.)
Reducing suffering caused by disease undoubtedly improves people's lives, but increasing the population without the ability to provide everyone resources doesn't. We know that simply having enough resources for everyone doesn't mean everyone will get enough resources. How we distribute resources matters and that's a social issue, not technical. Trying to solve social issues with technology doesn't work that well. Changing social patterns is much harder. Capitalism is great at increasing the pie, but poor at distributing it to those without capital.
I applaud his optimism and support for reducing suffering from disease and supporting education. I suspect he's helping create a world with similar fraction of people suffering versus happy, just more of them. And accelerating toward limits to growth, some of which we can't overcome with technology.
(If you challenge the belief that the planet has limits to growth or even that we're near any, I recommend the excellent blog "Do the Math" -- http://physics.ucsd.edu/do-the-math/post-index -- which looks at physical consequences to growth by a Caltech-trained physicist. I also recommend it if you like reading thoughtful, intelligent blogs by knowledgeable people who do the math behind what they talk about.)