I do not have any experience with an autistic child, so I maybe way off base, but, I think the best thing to do would be to be honest with him, but this is what I would do:
Acknowledge his fear and tell him it's okay to be afraid;
If it happens at school, a teacher will protect and help him;
We have sirens here, so if it came when we were sleeping we would hear the sirens and wake up and go to safety before it hit;
I don't know what would happen if we die;
If you lose all your things, you can buy new things;
If the wind blows him away, you would go get him.
I would also try to limit his exposure. My neighbors 9 year old is afraid of tornados too and has been for many years. She let her daughter help with the safety plan and stocking the safe room with appropriate materials. I think giving her daughter some control over what they would do when it happens, helped her daughter.