I am beginnning MC4+ this year with an 11 and a 12 yr. old. It is supposedly for as young as "4th" grade, under which your 9 year old would fall, technically.
I wouldn't have started that young, though; but that's just me. If he's reluctant, I'm concerned that MC may be too much. My son, almost 12, was/is also a reluctant writer, but has good form; my dd, 11, loves writing, but has one long, run-on sentence! If I had had him do MC at 9, I know I would have turned him off completely from future efforts of writing. I am glad I waited. I just continued daily copywork, started him on dictation 2x a week and began having him copy his Hebrew vocab. words down in his notebook (plus, continued oral narrations and good literature read independently). That's it, but with lots of groaning. Then last year, I increased it a little more, with a little less groaning to now no groaning at the end of this year; this next year he'll begin MC4+.
What other writing activities do you do? What about Copywork and/or Dictation? He's at a good age to start dictation. Also, a gentle intro. into grammar or just use the sentences you use for dictation to teach basic grammar concepts that make up a sentence is sufficient, too.
If you feel really concerned in this area, and would like a more structured curricula may I recommend you look at Writing Tales for his age? It encompasses copywork and grammar (though I don't think dictation, but has spelling taught in other ways) and teaches the sentence, etc. via class works of literature. It doesn't have as much writing requirements in my opinion. I think I could've used WT with my son when he was 10.
If you do WT 1 and 2, you have a few options to transition into depending...; there's Write with the Best, Jump In, Classical Writing if you want to go that route (though that's rigorous and may not be suitable for a child who doesn't care much for writng) or the later versions or MC.
After MC 4+ and 4, I may do 5+, but I'm not sure. Whether I do or don't, next will be Write with the Best. After the structured nature of MC, WwtB will be a nice balance, I think, then essay and research teachings in 9th+.
What do think?