Creating a path II

My middle son is not such a natural writer, he does not have such a natural affinity for words, he does not particularly like to read and seldom spontaneously writes. It’s not that he is reluctant, but just that it isn’t something he feels passionate about.
He is a funny and happy child and likes to put humour into his writing; he tends to write as if he is chatting to a friend and tends to put in jokes and asides. He does not delve much into ideas or expand on his topic but his is quite happy to free-write for 10 minutes. He can write (or at least print) very neatly and is happy to do copy-writing, although he would always prefer a shorter passage and it takes him quite a time. His spelling in freewrites is very original but he can usually spot and even correct mistakes.
I would like him to write in joined-up writing and then to work on writing a bit quicker but still neatly. I would like to see his automatic spelling improve. I would like him to develop his ideas and write more about one topic or point. I would like to encourage him to write about factual things (his interest in cars or what he learned about in his history lesson or how to play his favourite computer game) in a more ordered and logical, “followable” way.
I think this last goal is a good one to focus on. I think that talking through the topic with him, encouraging his ideas to be more step-by-step and slowing down his free-writing would be the way forward. Perhaps I will start with post-it notes, and then alternate sentences, then move on to him writing three or four sentences before I interject, or me adding words to his sentences to encourage more idea development and I think lots of good conversation before-hand will be key as I try to understand what he wants to write so that my writing makes sense and encourages him.