Re: the different religions... (been there!) explain that everyone with a religion believes in a higher power - we just use different words. For example we say God, Jehovah's witness say Jehovah, Islam says Allah, etc etc. You could make a list of cultures and names for God and then show DS. "See? We all believe that there is a higher power - we just have different names, and different ways of worship, but it's the same higher power." It's like different languages: dog, chien, perro, hund, cane, etc etc... it's all the same animal.
Most importantly, don't lie. Yes, people DO die. It's part of life. Make sure your DS knows that it's normal, it happens to everyone eventually, and it probably won't happen for a long, long, long time. It's ok if he's afraid, but he doesn't have to be. Show him you're not afraid.
My kids had fun with the idea of reincarnation. I told them that really, no one knows for sure if it's real or not. I told them that Atheists are not bad people, they just have their own beliefs. My kids are 8 and 10 now, and we've spent a lot of time discussing how it's ok for people to have their own beliefs.
My two are both very spiritual... I remember a carpool ride with a classmate when my DD was about 7... the classmate was a boy (HG) who did not believe in God (he wanted scientific proof). DD was horrified! "How can anyone not believe in God??" This led to discussions about respecting other people's opinions and beliefs.
My DD10 was PLAGUED by fear of death when she was five. It was really, really hard (I was exhausted stressing about it). Not just fear of death, but intense sadness about it as well. My sister had a cat who died when DD10 was 7, and she was traumatized for months (literally). The idea that a beloved could cease to exists was too painful for her. She's always been extremely sensitive that way. She was 9 and saw a dead bird on the school ground and it made her cry all the way home. Nine!! I don't think it ever really goes away.
DS processes differently... he stays happy and bubbly on the surface but underneath he'll think about it... for weeks. Out of the blue, weeks later, he'll start: "What happened to the cat's body? Is he in heaven too? Can he see us right now? Did he know he was going to die? Does anyone know when they're going to die?" etc etc... then DD gets mad: "Stop talking about it!!!" and starts crying again.
The key, I think, is be honest and share info with your DS in small pieces (don't share too much too soon). You might just have to ride it out until he's a bit older... my kids calmed down a lot about it as they've grown. They don't seem to have any atypical fear ("I can't ride a car - I could die") etc. In spite of DD10 extreme fear surrounding it when she was younger, she's full of life with a healthy sense of adventure. DS, meanwhile, is all fun. It does get better
