I don't know anything about kids so don't get me wrong, but how much of it is a 5 year old even gonna understand? Don't kids that age live in that blissful ignorance that is denied to us adults?
Generally when you're trying to convey and emotion to someone, the easiest way to do it is to mimic the emotion. If you want to come over as being confident about his safety, just put on a douchey smile and be like "That loser? Hah nah he's gone, he won't be hurting anyone, much less you. Wanna watch Legend Of Korra? Korra finally get's the metal out of her body!"
At his age I wonder if there will be all that much talk about it that he will hear. But assuming he will, I'd say go with as many of the facts as you think he can handle with a strong stress on certain aspects:
The person who did it is gone/dead and he can never attack your son.
It did not happen at his school.
(Assumption on my part but I think a fairly safe one.) You would only send him to school if you thought it was safe for him.
I don't have a child so if someone with kids says something which contradicts what I've suggested, go with what they say.
Realistically, I strongly believe there are other things to worry about even more. Statistically, even! For many (if not most) people, the big media stories contribute to an over-emotional reaction.
He's going to be affected more by other people's over-reactions than to the actual incident. Fortunately, I'm pretty good at being calm if not downright mocking at some reactions, and overblown media coverage is at the top of that list. The life lesson here (to me) is not about specific events, not even events like terrorist attacks, but how we all need to learn how to manage and deal with risks. All risks.
This may not seem like a positive way to look at it, but it's realistic. The chances of suffering and dying are higher with cancer, smoking, crossing the street, driving, eating fast food, or trying drugs from a stranger. Even mass hysteria and stampedes kill more people than school shootings! (Well, in other parts of the world, anyway.) And capsized boats full of people.
If there can be a positive side to this, I would be explaining to my kid how they might want to help other kids (and maybe even other adults) how not to be so upset about every tragic story on TV. From watching the reactions of other people in various tragedies, I think it's safe to say that the stress from such stories contributes more to our coping failures and mental health issues than the risks of the tragedies themselves to ourselves.
Sorry to go on and on, but another important (to me) thought popped up. I told my daughters to be very suspicious of any strangers that approach and engage them for any reason. But at the same time, I also explained the importance of finding a stranger if necessary to help you if you seriously need help with something. That's the way the odds of risks work. 99% of people at random are trustworthy. A significant percent of people who approach you with an agenda are not.
To me, life is about knowing which risks are truly worth worrying and doing something about... and helping others in the same light.
I'm sorry, I didn't consider how tragic stories have actually affected you, personally, and how you may still be at risk in certain circumstances. Still, overall, in the big picture, I'm sure I'm right. For your specific experience, there are other issues that also need care. If it means anything to you, part of the reason I didn't think of this before is because I'm impressed as hell at how you've been able to handle things in life, and didn't even think to worry about your future. Please stay positive, and keep passing it on! Others benefit greatly from your wisdom.
Has he actually been diagnosed with PTSD? My understanding was that it was rare for kids to have it