I was like this @Ainsley24 I remember vague fears about it even when I was little but they ramped up when I was around 10 I think. Having a best friend who's family were very intense Christians and hearing regularly about people going to hell certainly didn't help...
I'm 36 now and if I'm totally honest it's still a fear for me and I have to try not to let myself go there or I get a tight or cold feeling come over me. It's not just about myself either it's the thought of loved ones dying and some day everyone I know not being here or nobody who knows me being around. The unknown of what happens afterwards and the finality of it. Like almost every other Thing that's a fear you can conceivably avoid (in theory) but death is completely inevitable.
I never really told my parents how I felt beyond a general worry though. And I don't talk to others about it in real life now either because I'm too embarrassed.
Pretty sure people will be on in a minute to tell me I'm ridiculous, but it is what it is.
I'm perfectly functional adult though! Job, house, husband, child, hobbies. I just try not to think about it too much now. But it's there, in the back of my mind.
I guess just be supportive and listen and if you think it's getting to a point where it's really affecting him, maybe see if there's anyone he could talk to?