2. Tell him that an evil person walked into a theater, and shot a ton of people for no reason. If you believe in religion, now's the time to explain Heaven and death, if you haven't before.
3. Explain that he was caught, and can never come back and do it again.
4. Explain that theaters are still safe places to go to, and you shouldn't be afraid of that ever happening again.
5. (Optional) Offer to see the movie with him either when he's older (Because this is not a children's movie), when it comes to TV or in a few weeks when the price goes down and people stop freaking out over it.
Total time spent with child: 5-10 minutes. That's about the length of two or three YouTube videos, which is the extent of his attention span anyway.
It may sound crazy to think it takes just this little amount of time to explain such a thing, but keep in mind, I had to explain to a 9 year old (my brother) 9/11 as it was happening ~ LIVE. In the grand scheme of things, we end up explaining to children even worse down the road, so it isn't as hard to talk to your child as the media makes you think.