What I do is take dictation. That way ds can focus on his writing without his dexterity being an issue or the down right fatigue of hand writing more than a sentence or a list. Not to mention spelling. It works best if ds is composing something meaningful to him. Once, he wanted to write his cousins a letter. He dictated, I wrote and read it back to him. We had a brief discussion of whether some sentences should be in a different order to make better sense. Other times, he composes text for a computer game he plays. Some of it is description, some dialogue. He occasionally asks me to make it look like the speaker got suddenly cut off for dramatic effect.
I think this is the most important aspect of writing, getting one's thoughts on paper (or the computer, lol) and using writing for communicating. The details such as indenting for paragraphs (which I think is falling by the wayside and may well be obsolete by the time he is an adult) will come in time. Focusing on it prematurely will likely negatively impact the creative process. I don't think he needs to know how to do things like outlining this soon.
Here is a nice article I just came across...
http://www.nytimes.com/2010/02/02/opinion/02engel.html