Historically, the best way to lower the rate of increase in populations is to increase the economic well-being of those populations. In other words, the poorer the country, the higher the rate of population growth. Having said that, it needs to be noted that countries with the highest rates of population growth also tend to have the lowest life expectancies - which of course is a result of low economic well-being.
And I would tell my fellow countryman (is he really Canadian?) that the best way to preserve the natural world is to take people out of poverty. The poorer the country, the less people care about wildlife and wild places. It's difficult to worry about conservation of impala when you're worrying about the next meal for your family.
Lastly, I would note that China (which he mentioned) is regretting its one child policy these days. The demographic impact of that policy is potentially devastating to the Chinese economy in the long run - to the point that not only was the policy repealed in 2015, but China today is encouraging people to have more children - but with limited success (an example of one of the consequences - as I noted - of a stronger economy and people having options).