Collapsing fertility rates is because people are more prosperous. It is because children were once viewed as an economic resource to work the farm and take care of you when you become too old to work. If you want to increase the birth rate than reduce everyone to abject poverty. Move them out into the country and out of the cities where there isn't enough room to have a large family. Basically you will have to undo all the benefits of modern society.
You've restated the premise for the John Ringo book, _There Will Be Dragons_.
One "side" decided that the decreasing numbers of humans was a bad thing, and the way to increase it was to eliminate technology. The other "side" pointed out that the decrease in population had been slowing, as the only people who had babies were the ones that wanted to have babies, reinforcing the reproductive drive in subsequent generations.
Anyway, that aside, I thought it was quite a good book. Ringo is no libertarian, but that doesn't mean he can't write a good story.
I especially liked how, in the second book of the series, he cites the first book as if it had been written in that world, a best seller, and the characters now have to live with the fact that they're famous. I thought it was an excellent twist to be able to refer to backstory.