khpinson, one of the things I like about Time4Writing is that you're working with a live certified writing tutor. I believe if the level or writing course doesn't work for your child, she may be able make recommendations or adjust the assignments to your child. I love that the site has so many free resources. It's a wonderful resource to help you with guidelines in helping your son put his thoughts down on paper. Do you think that working on reading comprehension as you read books together would help him in his writing?
My daughter has several learning challenges. She is 16, but also writes at about a 2nd grade level. She has trouble with comprehension, putting her thoughts down, and sentence structure. In addition to her writing, she also has trouble with orthography/handwriting. Her spacing between letters and the size of her letters are inconsistent. I found a particular notebook paper that has been very helpful for her. This is the link: http://www.mead.com/meadstore/catalog/productDetail.jsp?prodId=48018
This notebook paper has a space for each letter. It's for 2nd through 5th graders. We LOVE this notebook paper. She's been doing a page of copy work a day. This helps her to see the punctuation, capitalization and spelling as she's writing. Maybe you could alternate between copy work and dictation. And then also have him keep his own journal for free-style writing. Just have him write 1 paragraph about something familiar to him, like what he did over the weekend, or how he takes care of his pet, or how he felt about a movie he saw, or what he would do if his best friend came to visit. :)
I hope these suggestions help. :)