We use a very solid grammar program and do most of it orally
This really helps my reluctant writer soooo much.
We use Rod and Staff English. No need to rush through....do a grade per year and then take half speed through 4 and/or 5....5 REALLY steps it up. Level 6 goes beyond the typical high school level (even though it says "grade #", I don't use that part).
I go through the lesson, which takes about 5 minutes. Then I do a couple sentences from each section of the exercises on the white board, we do a couple together, then she does a couple from each section on her own. Unless there's a worksheet, then she does that instead of writing the few sentences on paper. Total mom time is about 10 minutes, then 10m for her to finish.
Also, using studied dictation helps so much. You may also consider getting him writing by letting him dictate things to you (i.e. a narration, or perhaps a story, free time or assigned). Then he can spend a couple or more days copying it himself.