In Indiana, we have the Indiana Disability Rights(a state government department). You can apply for free advocacy services. Check if your state has the same and apply ASAP.
I'm newer to this as well, though we have found more testing is better. It is more proof of his deficits and could help strengthen his ASD level diagnoses.
My understanding is the ADOS-2 is not the best screening tool for high-functioning and/or highly intelligent children or adults. The MIGDAS-2 enabled our daughter to be diagnosed with ASD during her 2nd assessment. Her ADOS-2 score was a 6 during both assessments. However, her older brother also has ASD. The clinician admittedly scored her more conservatively due to her likelihood of modeling his behaviors. The MIGDAS-2 is more personalized and enabled her to confidently diagnose our daughter with ASD - Social-Communication: Level 1; RRB; Level 1
Regardless of his ADOS-2 score being 7, the IEE diagnosed him with Autism. I'm sure the school system is stuck on infinite replay of the....medical/clinical diagnoses is different from the educational. His issues at school are obviously stemming from his ASD and impacting all facets of his education, which should satisfy the educational diagnoses of Autism.
I would ask for a diagnoses letter from the Cohen Institute, if they didn't already provide you one. The assessment is great, but it's too much information for most of the CCC members. A diagnoses letter stating he has Autism(and probably other disabilities) and needs supports to receive FAPE. If they can list specific supports, that would be an even stronger letter. I would also ask for a similar letter from any and all of his current therapists(such as Occupational, Speech, Behaviorist, Pediatric Psychiatrist, and etc.).
If your son has a state case worker and/or a coordination of benefits case worker, ask either of them to help you advocate for your son.