I can't answer to the rest of it, but behaviorists don't just work with autistic kids- they often work at offices that may deal with those primarily but they do see other kids that have behaviors that are counterproductive to themselves and/or the school. They are like OTs but for behavior- they try to troubleshoot to find a solution. The behaviorist will likely know pretty quickly that your DS is not on the spectrum. However, if for some reason they don't, you can share anecdotally that several experts have stated that he is NOT on the spectrum. But I don't think you need to share anything with the school at this point, but telling them you have already had confirmation that he is not autistic will hopefully steer them into figuring out the right path (but only if you see them going down that path). But I defer to others who have actually gone through the IEP process as I have no experience there.
