Writing program needed. Recommendations please!

I need a good writing program appropriate for a very gifted teen. The boy is an OG success story, now reading with comprehension at post high school level. However, written expression remains an unremediated weakness.

Any and all suggestions/recommendations/first hand experiences, would be greatly appreciated. I’ve looked at the LinguiSystems website and Landmark School’s publication page, but find it most difficult to assess a book via the publisher’s website.