My son went through anxiety after his dad died - a lot of it centred around losing sight of me and whether I would die too.
First thought - Usually we worry more about what we have experience of rather than future unknowns. So is there anything that might have triggered the worry that’s close to home, like a change or death in the family? Or anything in a friend’s family? Is he perhaps fearing a similar thing happening again or to him? It might simply be exposure to the news that’s done it.
Second though - It would be useful to have conversations about the worry (and this means you being able to hold your own worry too and not be scared to go into scary realms with him). You could ask questions to follow the thoughts to their root and find out what he’s really worried about underneath (it will likely be something totally self-centred as that’s how children think - so might be something like a worry about what would happen to him and who would look after him if you were sick etc).
So then you could have reassuring conversations - ‘If mummy died, you might feel really very sad for a while. X person would look after you and you’d still have all your friends, teachers, school mates around. If mummy got sick, a ambulance would come to help mummy get better, x person would look after you, maybe mummy would have to have medication or be in hospital but you would be okay and safe and cared for. If there was a war in our country, we have an army to help protect us. It might be scary. We would try to get to a safe place. Mummy would look after you’… etc.
It’s about building a tolerance for worry, capacity to think through some possible outcomes and plan, and reassure him that it’s okay to feel worried and that it’s normal to worry. While not promising that bad things will never happen or that we should never feel worried.
Be totally led by him. Ask interested questions and go at his pace - if he’s not engaged or seems overwhelmed leave it and try a little another day.