Does your son have an IEP or a 504 plan? If not you need to push for one. An IEP under other health impaired if his education is being greatly effected by his ADHD and a 504 if it is not. This will give both he and you rights and protections under the law.
Unless he is causing bodily harm to himself of others, he cannot be expelled. I am a special education teacher who also has ADHD and who’s has a son with ADHD as well. I know it is a very difficult decision but I would consider medication. My son takes Vyvanse and I take Adderall (you have to find what is best for your child. I love that my medication helps
me to be able to focus and complete tasks. I am sure your son is frustrated by the fact that he cannot stop paying attention to all of the external factors going on in his classroom everyday and that frustration comes out as behaviors. I would like to say that it sounds like the school is working hard to help your son be successful. You might want to consider family counseling. It is very difficult to live with someone who has ADHD. It is normal to worry or feel guilty about your son’s behaviors but it is not your fault or his that he has a medical condition.