How has the author used Christopher´s alienating condition to expose the truths about our modern lives?

I'm not exactly sure what you mean here. For Christopher, his father's lie is completely devastating. He doesn't understand lies. Our modern lives are much the same. Truth has become ambiguous; protecting our children from things that aren't always pretty, and that are most certainly not truthful has become the norm. Secrets are unhealthy, and they always have been, but in the name of protection parents now twist those intricate truths so that they're not quite so upsetting.