Does he have trouble with his actual handwriting (ie messy, problems forming letters correctly, issues with fine motor skills)? If so I'd look at getting an OT assessment to see if anything needs addressing.
If handwriting is not an issue and it's more content, maybe allowing him to use the computer to do a short story every day (sometimes more appealing than having to pick up a pencil, or use a portable whiteboard something where mistakes can be easily erased and corrected). Start off slowly, just ask for a couple of sentences about what you did each day or what he would like to do tomorrow (ask him to use his imagination and come up with some fantasy stuff that he knows you couldnt do, but just for fun) and then get him to read them back to you. Maybe get a children's thesauras and come up with other words he could use instead of the simple words he has. Make a game out of it and see how many different words he can come up with.
Does he have a particular TV show / Movie or PC game he is interested in, maybe he could come up with a plot for a future episode? Or a story about the sports he likes to play. DS was a reluctant writer (both handwriting issues and content). He loved Pokemon so his teacher got him writing stories about Pokemon. You have to be careful though that the topic doesnt totally take over, DS had to be encouraged after awhile to branch out as every single story was about Pokemon for quite awhile.
I was clearing out some old workbooks yesterday and one of DS's teachers in primary school had them doing a simple sentence (I went to karate), then write the same sentence but add in one describing word, then write the same sentence but add in a conjunction etc so they could see the sentence grow from a very basic, simple sentence to a very complex one which conveyed a whole story to the reader. It wasnt so overwhelming then for them, they could see how just adding in one thing each time could really improve the meaning of the sentence.