Hi artichaut,
You imply that you have been trying to soothe him by various means, can I ask for an example?
I ask because faced with my children's worries about death and dying, I couldn't think of anything 'soothing' that wouldn't also be a lie/untruth/misinformation/require faith.
I think one problem is that many of us parents/adults in general, live in a fairly permanent state of cognitive dissonance with regards to death. We all know that death is unavoidable and unpredictable. And yet we live our daily lives as if we were never going to die, or as if we could be certain that our death/our children's death/our loved ones' death were far away in the future. Many routinely tell their children that we are not going to die, they are not going to die (perhaps qualified by 'until they are very very old' or some such).
I think that up to a certain extent, this is healthy too - we can't actually live normally if we are always worrying about dying. So we chuck all thoughts, considerations, feeling regarding death into a far corner of our mind, only to be brought out when actively confronted with death.
Some wise people might be able to hold the realities of death in their minds without being distressed/negatively affected by it.
But most children will go through phases of at first not understanding at all what death is, then becoming very anxious about it as the realities sink in, then learning that mental trick to 'believe' they (and their parents) are immortal whilst fully knowing that they will die one day, and that might be tomorrow. And eventually, perhaps, they will become wise and will perceive that death is not a bad thing.
Have you thought about/worked out what it is about death that is causing your DS such distress?
It may be the absolute lack of control, and unpredictability. (Does he struggle with control issues otherwise?)
It may be to do with abandonment and separation fears. (Does he struggle with separtion/attachment more generally?)
It may be existentialist despair, what is the meaning of life - stuff, tied to death. (FWIW, as a child when I had such an existentialist crisis, I more or less consciously tricked myself out of it. I asked myself, why am I here, what is the meaning of my life, of life in general anyway? Well, let's see, many people spend their whole lives trying to figure this out. Maybe that is exactly it- the whole point of our lives is to think about/figure out the 'meaning of life'! In which case, I've just done it. So now I am free to do with the rest of my life what I wish, without having to ponder the why anymore - ticked that box, so to say.)
It may be he has picked up a lot of 'Carpe Diem' type messages from popular culture and feels a pressure to live his life 'with a purpose', 'to the max', 'in today', ... and is thus worrying about potentially dying before having 'lived properly/in the way he desires/in the way he feels is expected of him'. Or another reason for being concerned about not living 'right' which could be terrible as you never know when life might be cut short.
With DS I had a lot of conversations revolving around 'sad, but not bad'. Yes it is sad when a loved one dies. But death is not a bad thing. Actually, if we want to give it a value at all, it is a good thing. There could be no life without death.
We also had many a talk about what the limits of our control mean for our daily, and big, decisions. Knowing that I might die tomorrow, or in 50 years, how does that affect my big and small choices? How do I want to 'be'? What is really important to me?
And further conversations where I made the issue explicit to him. No, we don't know when we will die, or what if anything comes after, or the 'why' of it all. There is nothing we can do about this. So how are we going to live with this? Are we going to let it worry us and make us anxious, or are we going to pretend it isn't true, or are we going to accept it and still find a way to live 'well'?