I find his fears are best handled by first, being very firm about that's not going to happen and you better stop this from coming. About 50% go away with authority threats. The rest are more difficult but I have never found sympathy to work beyond acknowledging that it is a real problem he is experiencing and it's not based on probable reality (which his aren't)
Yet, his therapist thought I was nuts. I agreed with her on the relaxation, positive self talk and similar treatment, but we parted ways on what to do when he was paralyzed beyond that. I was firm and she wanted to coddle the boy. He actually told me that the coddling made things worse, and we eventually had to leave her since she wouldn't listen to us.
If you coddle irrational fears, you admit they're real. I'm with epoh. That therapist didn't know much about unreasonable anxiety.
We don't make authority threats, but we work through likelihood of things coming to pass (very helpful that DS understands statistics). We talk about thinking mistakes like "catastrophizing" or "overestimating."
Also: we sometimes look at the worst possible scenario, and make a plan for dealing with it. If you are quite sure you have a path forward even in the worst scenario, you know you're going to be okay. (I don't know if that would work for all kids, but it is a known CBT treatment.)
DeeDee