I'm one stage behind you, same boat. I don't think regular curriculum is going to help you a whit. You need to move on to therapy-oriented materials. Winston makes grammar pretty visual, so you could *try* it. It's just not really the most important thing. Can he answer the wh-questions? If he doesn't have wh-questions for his everyday speech (ability to use them, ability to answer them), then he doesn't have that as a tool to harness understanding grammar either, kwim?
I've got Nurturing Narratives (aimed at autism), and when I first read it I thought oh that's way obvious. Then I watched my ds try to sequence 8 pictures and tell a story and I was like DUDE NO WONDER, kwim? Like we've got to go way farther back. Have you looked at his IEP goals? I notice how deftly our IEP team SKIPS having any actual, sensible IEP goals that reflect working on common core goals. Like look at Common Core gr 1 for writing. It says ability to retell a simple story, tell the moral of the story, etc. Can he do this?
I have some personal theories, but I'm behind you enough that my theories aren't tested or solid. My personal assumption or working theory is that my ds' writing development will follow a path, just like any other dc's, and reflect his oral narrative ability, just like any other dc. So when I look at what my ds is doing, he's going through steps my dd did, but he's doing them YEARS LATER. Multiple multiple years later.
So to me, I look at that and go ok, going to the end and wanting rising 3rd grade or whatever writing is just going to be a mess, because where his expressive language is, where his real ability to narrate and compose is, is like age, um, maybe 5 or 6? He might be 8.5, but he's functioning way, way younger. He's making lists. What age was my dd when she made lists like that, kwim? Listmaking is awesome and worthwhile. I'm cool with that. I think we're going to have to start some really basic sequencing with pictures and putting words to things. Maybe starting with non-fiction. Maybe starting with something high interest but simple. And maybe we make it more visual with powerpoints and stuff for the steps. I'm hoping to rearrange his office and bring in a low table, a computer for a writing center, etc., so he can work on this and be more empowered with his own communication.
Just seems to me if it's not in his mind to say, it's kind of pointless. And whatever is in his mind to say is going to reflect where he is developmentally and have no correlation to grade level expectations. He is who he is. I think I'm *more* likely to get more detail with something he's really into (something technical, something relating to his strong areas of interest). But really, when he goes into those, he's often scripting or gets this funky hollow voice. Like to me that's not real either.
If you want to try IEW, I would go low. He has really funny models, many non-fiction, so it might be engaging for your ds. Also Writing Tales could be worthwhile. I had it on my dream list to try with him at some point this coming year. I like the social component, that it's using fiction. I think it's something where we could really slow down and engage and make mental pictures. When I've done stuff with him so far, that's what I've tried to do, having him draw each scene and then put words to it. That gets you back to the Nurturing Narratives book. I think I thought that would just come, poof, like just happen if I waited, and now I'm realizing I'm probably going to need some strategies.
I really don't think you can go overkill on language work. Personally, I think cranking out specific assignments is less important than the overall language development. Anything you can do with playing games, nurturing oral narrative, etc., will probably nurture that in the long run.