Well, that is a belief of the Christian faith, so can't help you much on that end. I would want to know who was discussing Hell with a 7yr old. That is a bit young to be starting with that line of the faith.
Most kids less than about 10-12 cannot deal with that end of Christian theology. What you can say is that you(and him) don't know whether your wife had accepted Christ at any point in her life. I assume you didn't outright tell him that Jesus was a lie etc. If you are like most atheists on here you want your children to come to their own conclusions.
To be honest your best bet may be to involve the pastor of the Church he attended. If he is not listening to you about these things, he may listen to him. Talk with the pastor first and let him know what is going on, and tell him how concerned you are about your son.
He should volunteer at that point to help you with your son. Show your son as much love as you can. The things besides harming himself actually sound a lot like normal behavior of a child that age in losing a parent.
Counseling for the self destructive behavior may be needed too. You should also avoid slamming the church's beliefs in front of him, because that will just push him further from you when he needs to be as close to you as possible.
I'm so sorry, any person who makes a 7 year old believe something like that is evil. As a muslim I do believe in heaven and hell, but I associate heaven and hell more with deeds and actions rather than beliefs. Keep your son away from these people because clearly they have no sympathy about the fact that a little boy has just lost his mother and instead of consoling him they are instigating him.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.