Hm my DS is about a year older than yours, and has become concerned about death and dying recently (since Christmas). It started with his Gran's cat dying, and was not helped by the fact that our own cat is VERY old and pretty poorly, and we have been waiting for her to die for a few years already (she's tenacious!) so we cannot in any way reassure him on that front. Also his Greatgranny is very old and poorly and has been expected to die for a while now...
The thing about death and dying is that most of us adults deal with it quite irrationally. We pretend it is something that only ever happens to other people, or to people who have had a long and happy life, and completely shut out the fact that we ourselves could die at any time. This can be very confusing to children, as what they see, what they see about how we react to things, and what they hear us say, can be so very disparate.
I THINK that one way to go about it could be to stress that dying is not a BAD thing. It may be SAD but not BAD. It is part of life just like falling over and hurting one's knee is.
DS has been much reassured by 'understanding' that whereas I will die one day, and he will die one day, we will both be in heaven together. Most of all, his dinosaurs will come with him to heaven, so he needn't worry about other children playing with them.
(Now I don't actually believe in heaven, but when my toddler sister died when I was a pre-schooler, I found the concept very comforting. It kind of means that though we are sad, it's not 'the end of the world' as there is something after)
What I found to not work, or even to make things worse, was the whole referring to 'not for a long, long time' (when referring to his death, or my death). Thing is, a long time for him can be 10 minutes. Or 5 days. Or the immensely, barely imaginable long time to his next birthday. He literally cannot make sense of what 'a long time' should mean. Explaining it to him in terms of 'first you will start school, then you will finish school, then you will maybe marry and have children of your own, then they will grow up and have children of their own, then you will be very old' doesn't work either. On the one hand, it isn't really honest - he may not live that long, and he may encounter people dying at younger ages. On the other hand, he is still very much at a stage where he is constantly creating and rejecting theories about life the universe and everything. He comes up with ideas such as, when boys grow up they turn into girls, or, when he grows up, I will be small, and when I was small, he was grown up and looked after me. Then rejects them when he realises that reality doesn't match up. Then makes a new theory. That's how children do learn to make sense of things, but it means you can't take 'common sense' understanding of life and growing etc. for granted. Young children live very much in the present and can't conceptualise much about the future.
Also, now that we have tackled death, DS is very curious about where he was 'before he was in my tummy'. When I refer to something I did when younger, he asks 'where was I when you did that?' When I say it was a long time ago, he ask 'was it when I was a baby?' then I say no, before that even, so he asks 'was I in your tummy?' and then 'was I in an egg?' (DP has an odd sense of how to explain things...told DS that before he was in my tummy, he was in a tiny tiny egg) ... and 'you did not exist' just doesn't cut it! 'Nowhere' doesn't make sense, how can you be 'nowhere'? So, having gone with 'heaven' for after death, rather than 'vanishing from existence', we've decided to go for 'heaven' for before conception as well.
(Thinking about it, maybe we could move to 'in my hopes and dreams' for before life, and 'in my (or other people's) memories' for after death, to keep it a bit more secular)
All that said, what I try to do is answer his questions honestly, even if it means saying 'I don't know' to 'when will I die?' and 'Yes' to 'will I die?'. We talk together about how it is sad, and I don't say things like 'don't be sad' but rather, 'I know, it IS sad, isn't it.' But I do reassure him when he gets really upset (such as regarding other children playing with his toys). And try to get it across that it is a normal thing that happens to everyone.
I'm sorry for this being a monstrously long post, but there is one more thing I'd like to say...
At one point DS got distressed about the prospect that our cat will die. He was very sad. I said it was alright to be sad, and that I was going to be sad too. But she wasn't dead yet so we needn't be very sad yet, but instead, should enjoy being with her, and stroke her lots so she knew that we loved her. Hmmm that probably was a mistake. After this conversation, he stroked her all the time - not a problem... but he also started conversations out of the blue starting with 'Mummy I love you! Mummy, when you die, ...?' and I fear he might have gotten a sense that he can somehow influence/ward off dying by telling people he loves them. He now is very vocal about his love for quite a number of people!
Oh and regarding the worry about fires and dragons. A lot of children become worried/scared about something at some stage. Such as monsters - DS was always fine with monsters until all of a sudden he refused having monster books read to him, and will stop me reading if a monster turns up in a book where he hadn't expected one. I think it has to do with starting to be able to control/influence things and hence becoming worried about NOT being able to control things. And also part of the magical stage. You could try equipping him with a danger banishing tool; a verse to say (Dragon dragon on the wagon, one two three away with thee (or you)), or DS has a Gargamel figure who protects him from monsters... Also this is an age where many children begin to be afraid in the dark, so a night light may help.
HTH