When someone dies they no longer live on Earth but live in our memories.
Pretty easy one to go with. Talk about memories you have with your son, those are memories that are shared with the living. Then talk about memories you have with a grandparent or someone who has passed. Then explain they will always be alive because you remember.
There see, easy, no religion needed.
I am religious so this is not how I explain it to my kids but it is a part of how I explain it. I think it gives people peace and the ability to remember happy times without the guilt of mourning.