Overpopulation
The world is overpopulated, but that's just an average. It's more complex than that. Most developed countries have low birth rates, for example, Japan's is 1.0 births per women, well under replacement value. And most developing countries have high birth rates, like India's is 3 births per woman. They're on track to surpass China in population in thirty or forty years. As a rule, countries with more professional, educated, empowered women have fewer kids. So if you want to fight overpopulation, have more kids, train them to be teachers, and then send them overseas. 🙂
Back on topic, yeah, 15 kids is nuts, but I can't fault the guy - he's raising them, and they'll all probably grow up to be happy, productive members of society, working, shopping, paying taxes, etc. That's a net economic benefit to society, and who knows, if they help support him in retirement, it may be a net economic benefit to him also!