My DS sounds very similar. Despite having an amazing vocabulary and imagination, he struggled to get his thoughts down on paper throughout the 1st and 2nd grades. We tried all sorts of story webs (early outlines), etc., but nothing seemed to work until we tried IEW (Institute for Excellence in Writing). -- By the way, he is a movies in my head kid.

Anyway, IEW helped him tremendously. It gave him a step-by-step method to get his thoughts narrowed down and onto paper. And, now that he's been using it a couple of years, he has found he can skip several steps without impacting his writing speed or ability.