How is his written expression when allowed to use assistive technology? E.g., speech-to-text, keyboarding, spellcheck. There are numerous possible sticking points for writing, some of which are mechanical in nature (handwriting, letter formation, spelling, writing speed). Other aspects include idea generation, initiation, organization, and elaboration. You can try to scaffold various aspects of writing diagnostically, to see which kinds of support are most effective at helping his writing to approach his oral language.
