His son did not die. Listen to what he says outside the farm.
[Paraphrased]
"He couldn't breathe in the poison, he couldn't breathe in the poison, etc... ..."
[/Paraphrased]
Things happen for a reason, that was what the movie was trying to say. He thought the his son's asthma attack was unfair, but it turned out to be a blessing in disguise.
Hee, hee... just because I nitpicked on the alien invasion part, does not mean I was not paying attention to the main plot of the movie.
I love the movie.