Sounds like it lines up quite well with Polarbear's experience.
The nice thing about bright people is they often have an innate aptitude for taking control of their own meta-cognitive development. For a kid it may just take a little prompting to get them thinking about building their own toolkit of tricks and skills to get past their stuckednesses.
I'd definitely suggest a picture prompting approach as one element. You can make a set of a dozen or so pictures from magazines pasted on index cards. Another creativity skill that can help bridge from the analytical thinker in to more creative approaches is an oddly mechanical approach: create categories and sub-category structures then mentally step through those until something clicks.
There can also be blocks when someone isn't sure that they understand the system they are supposed to be working on (e.g. what are correct ways to make a story.) Theoretical reading on plot structure (e.g. the 7 basic plots), narrative pace may all seem to be high schoolish in knowledge content, but may be the sort of content keys a bright kid needs before they are comfortable diving into such a nebulous ill-defined domain as creative writing.
A final element for the moment is that having any sort of randomized input (e.g. I've used the milliseconds digit on a watch/stopwatch) can distance him from the weight/responsibility of making a first decision before writing. "I will write about either 1-3: Monkeys, 4-6: Cars, 7-10: Robots."
There are lots of techniques, I don't know what OTs do or suggest.
