How flexible (or inflexible) should I be about what my son reads?

My 14 year-old son is a voracious reader. He reads lots of young adult literature (he loved The Maze Runner trilogy, and the Graceling series), and has also read plenty of adult fiction from This Boy’s Life by Tobias Wolff to most of the Lonesome Dove series by Larry McMurtry. Now he is interested in learning more about World War II and the war in Viet Nam. He asked me to get him a copy The Things They Carried by Tim O’Brien. I want to encourage him to read and continue to be exposed to lots of great books, but some of this stuff might be pretty intense for a 14 year-old.

I have always let him read what he wants (aside from smut, which of course he will read behind my back if he really wants to) and I am wondering if I should continue to do so or if I should have him wait until he is a bit older?