Hi there, we used to have similar issues with ds1 (he is now 10 and has AS). Vocabulary is actually one of his biggest strengths and he was always full of ideas, but he struggled to get anything down on paper - at most we'd get two paragraphs.
I found Xanthippa's blog really useful for getting my head around what the problem was.
In ds's case it was mostly perfectionism. He hates to get things wrong, but also hates to revisit and edit and of course anything written by hand is difficult to alter anyway.
We've found the problem has slowly improved since he learned to touch type and is allowed to use a pc for school and homework.
The other thing that's helped is approaching the 'creative' part by sort of mind-mapping it. So, drawing big circles on pages with either pictures or words for the ideas, then arrows to indicate connections or progression of the plot and eventually filling in some of the connectives along the arrows. He can't do this alone, but it definitely helps if we do it together. Basically, we get the bare bones of the plot as just a few blobs on the page, then start fleshing it out and scribble down any great phrases he comes up with while we are doing this. When we get that it seems to become less scary and once he starts typing he can usually keep on flowing.
He absolutely hates doing the school style planning of opening, middle and end and if I mention 'drafts' and 'final copies' panic and temper set it, so it has to be very carefully handled.
500 words is a lot and I can see why that is making things even more daunting for him in terms of getting started.
Interestingly ds's teachers also insist he doesn't struggle with this at school, . he's in the top group for literacy, but then he is never expected to write more than a page, uses cloze worksheets 99% of the time and is now allowed to use a pc anyway.
Having seen his exercise books, I have to disagree with his teachers - not least of all because several of the 'stories' he's written have been practically word for word re-writes of programmes he's seen on TV. 
I told his teacher this once, in relation to a piece of work she was telling me was amazing and clearly demonstrated he didn't have any problems there. It was word for word, move for move an exact transcript of the Pokemon episode he watched the evening before - even down to the characters' names, however that didn't stop them grading it high and holding it up as an example of what amazing 'creative writing' he can do.
