Is the story all his own work, or is that the final copy, after you've sat down with him and gone through it with him? If it's what he's just written by himself I think it's fine for his age!
Do you know those Usbourne 'First experiences' books? There are heaps of them - they have a little yellow duck hiding on each page, you know the ones? They have a simple sentence at the top of each page, and a simple paragraph at the bottom. Presumably so that learners can read the easier sentence and then the parent can read the paragraph - the learner gets a sense of achievement, while the detail of the paragraph keeps their interest. Anyway, how about you use this method when you sit down with your DS and work on the story together? Break it down so one or 2 of his sentences are at the top of the page, he can draw a picture in the middle, and then you can scribe a paragraph going into greater detail. Something like this:
'There’s a dinosaur called Eddie.'
*Picture
(then you write)
'Eddie lives... (Where does he live DS? Oh, in the woods? What's it like there? There're lots of trees? And a pond? How big is the pond? Really big? Ok. How about we use a special word like MASSIVE or HUGE, that would make the story sound extra exciting wouldn't it?) '...in the woods where there are lots of trees. Nearby is a HUGE pond.' (What does Eddie think about the pond DS? He likes to swim in it does he? What about the other dinosaurs, do they use the pond too? Ahhh, ok, they're afraid of the water?) '...Eddie likes to swim in the pond, but the other dinosaurs are afraid of the water.'
Next page
'It is gigantic and nice to animals.' (Let's say 'Eddie' instead of 'It' shall we DS, now that we know that's his name?)
'Eddie is gigantic.' (I love the word 'gigantic' DS, it's much more interesting than just saying 'big'! Maybe people would think a gigantic animal would be scary to animals, so we could say 'Eddie is gigantic, BUT he is nice to animals?' What does he do that's nice? He plays with them? Eddie likes playing in the pond doesn't he? Does he play in the pond with the animals, or are they afraid of the water, like the other dinosaurs? Oh, they aren't afraid when Eddie is there? If you were one of the animals, how would you play with Eddie in the water? Maybe they could sit on his head and then whoosh down his neck like a waterslide, what do you think? You think that sounds fun? And he jumps up and down to make big waves for them? That's a super idea DS!))
'Eddie is gigantic, but he is nice to animals.'
*Picture
'They play with Eddie in the pond. The animals aren't afraid of the water when Eddie is with them. They slide down his neck into the pond, and he jumps up and down in the water to make waves for them. It's very exciting.'
Next page
'It seems to be superb kind.' (Well, we KNOW that Eddie is super-kind don't we DS, so we write 'Eddie IS super-kind')
'Eddie is super-kind'
*Picture
(We know that Eddie is FUN because he plays exciting games, but what does he do that's KIND? What do your friends do that's kind? If you're sad they tell the teacher? Eddie is quite big isn't he, maybe he's like the teacher, and looks after the animals if they're sad - what makes them sad? If they hurt themselves? Maybe they sometimes hurt themselves in the pond? I KNOW, suppose they swallow the water by accident sometimes and that makes them cough, and then Eddie looks after them? You think that sounds good? Ok, how about this:'
'Sometimes the animals swallow the water by accident, and that makes them cough. Eddie looks after them until they feel better.'
(What's a good way of making somebody feel happy again DS? A hug? Definitely! How do you think a dinosaur gives hugs? With his big long neck? What else can he do to help them feel happy? How about jokes, or funny faces? He tells them jokes, but no funny faces? Ok.'
'He gives them a hug with his long neck, and tells them silly jokes. Then they feel happy again.'
(What happens next DS? They all have dinner do they? I know what we could do then - because we already know the dinosaur is called Eddie, we don't need to write 'I think I'll call him Eddie', we could write about them having dinner instead!)
'Eddie has dinner with his friends.'
*Picture
(What does the dinner taste like DS? Nice? We've already used the word 'nice' in your story haven't we. Can you think of an even better would to describe something that tastes good? Maybe 'tasty', or 'wonderful' or 'delicious'? You like 'delicious'? Ok!)
'The dinner tastes delicious.'
(What do they eat DS? Different dinosaurs ate different things. I think Eddie is probably a 'Herbivore' that just eats plants, or the animals would probably be afraid he would eat them, what do you think? What sort of plants would he eat?)
Blah blah blah, you get the idea! It's a tricky line to walk between not wanting to put him off the whole thing by being critical and making him 'work' (writing stories should be fun!) and helping him to see how they could be improved without taking over. If you can spare the paper make a physical 'book' out of his stories with the pages stapled together, or fastened with those paper binding tag things. Keep it on his book shelf and read it with him sometimes. He'll feel proud of it, and hopefully motivated to keep writing. I loved writing stories with my mum when I was little.