Is his IEP for SLD/Written Expression or for something else? If it's only for written expression, do you think he needs instruction? If not, you should be able to continue his accommodations for dysgraphia in a 504 plan.
Our ds did qualify for an IEP (SLD/Written Expression) using the discrepancy model - he had a larger (much larger) gap in ability vs achievement in written expression on the conventional achievement tests.. and even then he didn't technically meet the official cut-off to qualify. What helped us most then (and continues to help as we continue to advocate for new/updated accommodations) is what was mentioned above - having examples of his work with and without accommodations.
It also helped us to always bring a brief write-up from a reputable source re what dysgraphia is to every meeting we went to because inevitably someone from the school staff would bring up the point "but his handwriting *looks* fine".
