Please, whatever you tell him, don't use the line about people only dying when they want to die. I know I'm repeating what others have said, but that is so potentially damaging - of course you have to soften or simplify or even bend the truth for a young child, but I can hardly think of a worse thing to say to him, especially if he's a literal thinker.
Hopefully chances are that you or his Dad won't die for a long, long time and long past when he'll understand the truth about death (insofar as anyone does). But you can't guarantee that, and just imagine how abandoned he will feel if you himself have told him that you won't die until you want to, and then you do. All bereaved children feel abandoned anyway, but you will have actually told him that yes, you really did want to leave him. How awful.
I lost my mum when I was young, of a disease that she fought tooth and nail to beat, and I still in my more wobbly moments have to remind myself that she didn't want to leave me and not see us grow up, or be here for her grandchildren etc.
Not to mention the other conclusion your little literal thinker might come to, that he can run through traffic with abandon because he doesn't want to die.
And you'll surely be asked why everyone tried to save poor Molly when she obviously wanted to die anyway :)
Maybe try something along the lines of "people die when they're very very old, or very very sick, or when they have a terrible accident like being hit by a car." I think Himilaya's suggestion for illustrating the 'very old' bit is very good, and then you can reassure him that as well as being young, you are very healthy and very careful, and you want him to be healthy and careful too, which is why you make him eat veggies and sit in a car seat etc. :) I'd say he's looking for reassurance as well as a sense of control, so while you can't (or really, really shouldn't) give him any promises that he won't die until he wants to, you can make him feel safer and more in control of his health.
As far as what happens when we die and what happens after, you can tell him that it doesn't hurt, because when you die your body doesn't feel pain anymore, and I think it's fine, as you're agnostic, to tell him that lots of different people believe different things about what happens to us after (maybe even give the light version of what some of those beliefs are), but that the one thing we do know is that life is energy, and energy can't be destroyed, only changed. So we don't disappear, it's just that our energy changes. But we don't get to find out exactly how until we get there ourselves.
I hope he stops worrying about it soon.