Hi
I was a KS1 Teaching Assistant for ten years; now a voluntary helper in a small Primary school two days a week.
Probably all children find writing much more difficult than reading, and I guess there just isn't time in the curriculum to develop and extend writing properly. There is not enough time to go over pieces of writing in depth and detail so that a child understands where they are going wrong, or how their style or ideas could be improved or extended.
And there are so many different sorts of writing required by Yr6: story telling; descriptive writing; report writing; recounts; writing-up experiments, investigations, data collection etc; poetry, and more. I am a trained touch-typist, and with reluctant Yr2 writers years ago I sometimes used to type to the child's dictation; boys who seemed to have no ideas and wouldn't hand-write much, could dream up brilliant stories when I typed it for them!
I have never been a much of a 'creative' writer myself, but am OK on factual or report writing. Even now, working with KS2 children I have little to contribute to help them with the descriptive or imaginative material, but I am fine with the 'technical' aspects - punctuation, etc.
As your son reads well, maybe look at his favourite book with him, and talk about the style of text, how could it have been put differently; why is it punctuated the way it is; re-write it keeping a similar meaning, but in a different style: journalistic, scientific etc; write it as a celebrity, pop star or teenager might write it.
Modern children's literature embraces a vast range of styles, so see how many he (or you) can find. Depending on your relationship with school, ask his teacher/Lit Coordinator to suggest books for different genres and writing styles; or ask your local Library for ideas.
Again, if DS is amenable, suggest he writes something every day - even if only a line or two - like a diary, but putting down anything he wants, news, ideas, worries, hopes, friends and relationships. Obviously, you can only do this if he is happy about it, and maybe focussing too much on his (lack of) writing might make him worry about it more, and be counter-productive.
A brilliant book I read recently is "Eats, Shoots and Leaves" by Lynne Truss, about using punctuation; it is for adults, but she has also done similar ones for children. I'll try and give you the link, but I've never done one on here before so it may not work!
www.lynnetruss.com/pages/content/index.asp?PageID=8
Cheers, and good luck.