Lynn,
Children are all so different as far as comfort in writing goes. One large difference in how easily kiddos "take" to the written page is how much oral narrating and how much good literature they have done and read in the years prior to being asked to write much. For kiddos who come up through HOD, we include both of these items in the guides making it easier for the kiddos to be ready to write once they get to the guides that schedule a more "formal" writing program. For those children who come into HOD later, we still include these helps, but it honestly takes time to develop those skills so that is to be expected.
As far as written work goes, from my years in the classroom to my time spent on writing at home, I'll say that it is a fine line on how much help to give children with the writing process. It's important not to give so much help that the composition no longer resembles what the child wrote and instead becomes yours.
However, it is also important to give the child enough help that the writing assignment does not go on too long or become overwhelming.
With kiddos in my classroom, in my public school teaching days, we did a lot of brainstorming on a white board to get kids thinking. Since your son has some struggles in this area, I would do much of the brainstorming with him, even allowing him to dictate his sentences for you to write as his scribe. Then, you could give some suggestions and guiding questions and help him do a bit of adding or subtracting. The goal is for kiddos to feel successful and not be overwhelmed. Being fearful of the written page, or feeling like the child isn't doing it "right" is something we want to avoid.
I know this answer goes in circles a bit, but the balance in how much help to give is a precarious one. I tend to give more help than not enough, unless the help is met with resistance!
Then, I back off from sharing my ideas and thoughts and let the child go.
Blessings,
Carrie