The law protects kids with disabilities from punishment due to symptoms. No one should threaten expulsion. In fact, them planning an IEP meeting is a sign that they want to work with you and help your son, not the opposite.
Here’s some information on IEPs and the law for you to review to understand what is taking place better:
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism