Was it the school neuropsych who did the diagnosing or did you bring him to someone outside of school? If this was done outside of school, the school will only have that as part of his records if you provided paperwork from the diagnostician.
It's possible that the school records are sloppy or are missing the fact that his doctor tweaked meds to help him have better behavior. (Maybe they only looked at school records & not things you provided.) Under FERPA, you are allowed to clarify school records. You might want to see these records to determine if you should add something so they know he's taking meds for behavior and as long as they are right, he should be OK to be in his neighborhood school & not an alt placement. (Not sure if you should email them as to why they feel he might need an alt placement as you have not been informed of any issues he's having at school. I've seen where parents are not in the loop so the school can have data where they should be keeping parents in the loop so meds can be tweaked. I know as kids grow, dosing can often need to change.) If the person is new, they might not know your son and the fact that things settled down.
Has the school requested parent input with doing the triennial eval? You could mention that he's taking meds to help with behavior and you want to know of any issue the school is having with him so meds can be tweaked when needed. I know that my son who has ADHD needed meds for both focus and to cut down on impulsive outbursts. Luckily he was OK at school. I think video games were a trigger for him. We did adjust how much he was getting from time to time.