If I understand correctly, he is in a substantially-separate placement for children with special needs? Or is it an inclusion placement (both NT and special needs) that happens to be heavily-weighted toward special needs? Also, if the school is proposing to dismiss him from an IEP, I assume there was recent testing done, including cognitive, achievement, executive function, and behavior. Change of placement (including termination of services) cannot be done without a re-evaluation, which you would have known about, since a parent's signature is required. If there was a re-evaluation, and you are not satisfied with the evaluation, you can request an Independent Educational Evaluation (IEE), usually at least partially at the school's expense (depends on the state; some have an income test and sliding scale). And you can appeal the finding of no special needs up the state chain, if you feel your documentary support is strong enough.
With a doctor's diagnosis of ADHD already in hand, he also qualifies (if the IEP route doesn't work out) for a 504 accommodation plan, which won't modify his instruction much, but sometimes helps with organizational and attentional supports. And offers civil rights protections in the event of certain questionable disciplinary actions.
How is his report card? Are there deficits of any kind being reported by his classroom teachers? The educational impact test in the IEP eligibility flowchart is not supposed to consist solely of standardized test scores. (Unless they are proposing to instruct him entirely in quiet one-to-one settings, clearly, tests won't predict performance in busy classrooms very well for EF-challenged kiddos.)
