If you want to get him to write random stories, you could try story cubes too.
What DS does often is to take a story he knows (e.g. latest bedtime story: James and the Giant Peach) and change it. So new main character name, make it about a giant apricot, and adapting the storyline, usually by incorporating bits and pieces from other stories he knows.
But I think the story cubes, the pobble website, and the 'adapting stories' are all strategies to promote imagination, story telling, becoming elaborate and detailed etc. but less about writing per se.
And I think it is only natural that many children (and grown ups!) can't get everything they imagine/think onto paper very well. Writing can then feel very frustrating, your writing speed often can't keep up with your thinking/imagination speed.
So if you think he has a great imagination and comes up with good stories and is articulate, but just doesn't bother to WRITE carefully, and that is what is bothering you, then I agree with PP that it would be worth checking if he can't or won't. If it is that he won't, and all you are looking for is 'how to get him to see the fun of writing' and 'develop a desire to write', then I think it is important to acknowledge that for the time being, whereas inventing stories may be fun, writing itself may NOT be. In fact, do YOU write for fun? Most people will write for purpose. Writing is often a chore. If writing is fun to you, what makes it thus? Can you share this with your child?
To see the fun in writing:
Perhaps look at poetry with him? How to convey big ideas/feelings/atmosphere with only a few words?
Perhaps teach him a simple cipher and let him write coded messages to his friends?
Or similarly, make some invisible ink for him to write messages to his friends?
Have him and you both write a 'diary' of a particular day/event and then compare. Did the same things stand out?
Writing can be fun (to me anyway) because you get to play with words. You could make up silly Dr. Seuss style rhymes with him and write them down?
How about writing a 'joke per day' and giving the resulting joke collecting as a gift to a family member?