I like the idea of using play to talk about what's real. This is definitely what we do at our home. When my son became interested in Star Wars and thought Darth Vader was 'cool', we explained that he did a lot of bad things and that while the toys look interesting (he's not interested in watching the movies after seeing a small, mostly fast-forwarded segment), in real life, this would be a horrible bad person. We've told him the same thing about pirates, that we know that kids think playing pirates is cool, and pretend pirates are okay. But that there are real pirates who do steal and hurt others and use weapons and scare people to take what they want.
One thing I would say, though, is wait until your son brings it to you, if you know what I mean. Wait until he begins to talk about and ask about those concepts. Before then, if he's not interested, it's just one of those strange forced conversations... Be aware, too, that if you offer weapon toys, they ARE going to dictate the storyline to most likely include weapons/fighting play. There's no way to really say "here are some things which hurt other people, but don't play like that".... kids need to try out these ideas in a safe way, which is why we put boundaries on it (no pointing toy weapons of any sort at any living thing) and then let go somewhat.
The best ways to plant seeds of non-violence are to model this, to offer good stories and media which promote mutual respect, caring and problem-solving in thoughtful ways. I personally think that some parents do tell their children about too, too much of the world and place large burdens on their kids early on in some conversations. Be careful to just answer his questions when they come up, but not to offer information that he might not be ready to comprehend. (One person recently told me that her first grade son knew 'all about the Holocaust'.... what a huge mind-blowing confusing thing for such a young person.)
Keep it simple, answer the direct question, and keep those conversations going. I often find that the best information I can get from my son comes from one simple question: "Who did you play with at recess today? What did you play?" I've learned a LOT about what the kids focus on at school and ask him daily. To him, it's far more interesting to talk about than what was taught.:)