I would assess him on Six Traits each way (oral and written). If he scores poorly both ways, then it suggests an OWL LD. If he scores well orally, and poorly written, then it documents written expression difficulty as distinct from language. If he does well on written work with accommodations, as compared to poor independent written work, then it documents the value of the accommodations. The rubric is not a norm-referenced instrument, and thus does not have the psychometric and statistical robustness of a good NRI, but it does have the benefit of being more familiar and comprehensible to educators, and more clearly tied to aspects of writing instruction. It also falls into the general category of curriculum-based assessments, since 6T or 6T+1 is a part of quite a few curriculum frameworks. Hence RTI-friendly. You might even request that a grade-level teacher or curriculum specialist (preferably both) use the rubric to score writing samples (blinded to condition, obviously). That might require having someone re-type all the final products so that it isn't obvious from the handwriting when he's being scribed.
