My son is being removed from class every time there is a substitute teacher

My son is on an IEP, educational autism and adhd. It is not in his IEP to be put in another classroom during substitute teacher days in his classes. He is being removed from his classroom every time there is a substitute and placed in another classroom. My son is 12, 6th grade, been on the IEP since 3rd. I am arguing with the school asst. principals on him being returned to class during those days. They initially said it is a sub plan he is on, in email they referred to it as a success plan. First I thought I was part of the IEP (my fault for not double checking) and told me I couldn't make that change til the IEP meeting we are having soon due to triennial evaluation. (They are testing now.) The lady in charge of IEPs at the school confirmed it is not part of the IEP, it doesnt need to be amended. I followed up with another phone call and never got a response to the message I left so I emailed the assist. principals and told them he needs to be returned to class during these days, its not part of his IEP.

My initial research into sub plans is that they are for the substitute to have direction, not for the student. My brother in law who taught for 10 years confirmed this and sais he's never seen it be a part of a 504 or IEP. They said that they won't send him back to classes during those days bc it's a team decision and needs to be addressed during the IEP meeting. Basically they're telling me to get f**ked. I contacted my IMPACT cae worker, she said if it's not in the IEP then he doesn't need to be removed from class. I told the school that he needs to be allowed to attend class during those days, as the purpose of the IEP is to mainstream him to being able to function in normal class settings and this is missed class time. Again, no dice.

My son has been asking us to take him off the sub plan, to which the more my wife and I talk, the less sure we are that we were ever informed about this. Now I'm confused and royally pissed. I don't care to wait weeks for the meeting to address something not part of the IEP. I have no clue what to do and I'm feeling quite powerless and angry. They're ignoring his wishes and ours. What do I do???

Edit for info: I am his father. My son does not have violent tendencies at all. He had one incident where he hit a kid for making fun of his stutter repeatedly. That was in summer school 2-3 years ago. He shuts down when overwhelmed. He has had subs in the past. He has had subs in SPED. To our knowledge we were never informed of this change until he brought it up not wanting to do it anymore. When he shuts down it usually ends up with a asst principal coming down and coaxing him to their office. We have been reassured repeatedly that him ending up there is not a punishment nor that he is in trouble for it. He also has all As and Bs, he does have problems finishing work and staying on task. He is a motor mouth and has tics that are in his IEP. He is also participating in the schools talented and lifted programs, has been since it was available. He tested out at an 8th grade reading and comp level, in the 6th grade.

We just had the triennial and are awaiting the follow up meeting after retesting. I had to force the school to restest for new data as they tried not to.