I would still go with OMT; diagramming is a very useful tool and that alone will help with writng. If there’s over-kill, you don’t have to do every single problem, but I would still encourage you to have him learn diagrammming.
Secondly, if he’s used IEW before, then perhaps go back to that and learn these composition skills.
I second starting him on oral narration. Start with one paragraph at a time and move to greater amounts as the year goes along. Don’t rush this skill, but do expect effort (which will be resisted, as it’s hard). It forms and trains the mind in an amazing way.
Personally, I had issues with having my child do written narrations without some formal teaching of the structure of writing; it was like walking before crawling, IMO. If he needs formal teaching in all the composition style and analysis, then I’d go back to IEW. But just to give you some options to look into and then about about more specifically (to find out if they cover all he needs to know) or look-up past posts anmd reviews at Cathy Duffy:
Jump-In – http://shop.apologia.com/35-jump-in
Write Shop – http://www.writeshop.com/
Writing Strands – http://www.writing-strands.com/
Skills for Rhetoric: https://www.forsuchatimeasthis.com/language-arts/junior-high/skills-for-literary-analysis
Write With the Best Vol. 1 and 2 – http://www.edudps.com/
First, I would recommend finding out, for sure, what he needs over the next four years and then going backwards from there to determine what to use in what year. That’s what Sonya’s planning series does. It gives you a big picture layout so you can see where you’re going; not just what your doing this year. Being that he’s 14 and in 9th, I would definitely look at this year as part of four and not just as an isolated year like one can do in the younger years.