Our experience has been quite similar to yours. Although we have never been able to get an official diagnosis other than "other physical disability" for our DD13, we now have a 504 in place due to her VMI of 6th percentile. (She is a straight A student in accelerated classes.) What helped for us was having our district OT come to her determination meeting and say "If this child were an EC child, I would be serving her immediately." We also advocated on the basis of her needing accommodations officially in place for when she takes AP English and other AP courses in the future, as well as for the writing portion of the SAT.
Best of luck in advocating for your child as he returns to public school.
