Well I would be careful saying that it's not going to happen... because, well, he could die tomorrow... (Sorry, I know it sounds insensitive & horrifying to say it that way, I don't know how else to phrase it!!) I'm sure your DS has some concept of this, on some level, so it may be confusing to him to just brush off his fears by saying it's not going to happen.
Part of life is death & it has always been important to me to help DS understand that death isn't some mysterious thing to be feared (or something that will never happen to him) but just part of the circle of life, and that all living things do die. However, I also make sure to assert that MOST people live to be very old.
It might be interesting to ask him what scared him about dreaming he died. Was it the actions leading up to his 'death'? Did it seem painful? Is he worried he isn't going to see his family & friends again? Is he afraid of floating out into the universe? And once you understand more specifically what he's afraid of, you may be able to use facts plus your personal/spiritual beliefs to alleviate his fears. For example, I explain to DS that we will see each other in Heaven. He also wanted to know what would happen if DH & I died, so I let him know who his guardian would be, and that all our mama friends would also look out for him, etc.
As far as nightmares in general, usually I remind DS that it was just a dream, and dreams are pretend. I have also found it really helpful to check in with him about his good dreams too. Now he runs up to me in the morning and tells me all about his good dreams. Sometimes he talks about his nightmares too but he is no longer losing sight of all the good dreams he has. Another thing we did when he went through a really bad nightmare spell around age 2, was a little activity right before bed: We got a box, and I had him put all his scary dreams into the box & we'd lock it up together. Then I put my hands on his head and pretended to send in good dreams and happy thoughts. This seemed to help a ton.