This must be very upsetting for your and your ds. I've been there with my ds1.
I'm not sure I can be much help, but wanted to bump for you and thought I'd tell you what we did to help our ds.
Also, I know that its around the age of 5 that children start to become more aware of the real meaning and significance of death. Obviously this is playing on your ds's mind, hence the dreams.
My ds used to have some really frightening bad dreams (probably starting around the same age to be honest - he is nearly 8 now). I talked to him about how dreams are just your brain keeping itself busy when you are asleep and that it tends to think about things you don't like thinking about when you are awake and that's what causes the bad dreams. I then explained that if he had any worries, about anything at all, he could talk to me about them and maybe I could help him sort things out so he didn't need to worry about them.
I did tell him that I couldn't stop him having bad dreams, but that we would try doing some things that 'might' help.
First I bought him a really colourful dreamcatcher (off ebay) which was to try and catch his bad dreams. We hung it above his bed and then hung it in his window when he got up in the morning to make any bad dreams it had caught disappear. We also started to recite a little poem from one of his books called To Sleep With Angels every night before he went to sleep. We aren't religious, but children seem to take comfort from the idea of angels so we used that to help him feel safer.
The poem goes
Angels, Angels all around,
Angels, Angels come surround,
Hold my hand throughout the night,
Guide me from the dark to light.
Ds changed this to "Hold ds1's hand throughout the night" and later extended it to "Hold my family's hands throughout the night, Guide them from the dark to light"
At the end of the poem he always says "Happy dreams tonight please Angels, thank you!" That bit was his own addition.
Another thing ds does after he has had a bad dream and can't get back to sleep, is imagine a big circle of angels holding hands around the house and filling the space inside the cirle with golden light. This helps him feel safe again so that he can go back to sleep.
He does still get bad dreams occasionally, but copes better with them now he feels safer and more protected.
I'm not suggesting you do the same as us, but wonder if maybe helping him to come up with some sort of comforting bedtime ritual would help him to feel safer so the dreams won't feel so scary.
Hth