If it's the loud music, have him wear head phones. My grandson is in special ed and has difficulty with noise. The classroom has several sets. His mom and I went with the class to see a kids play in a large building. He wore the headphones and could still hear the actors. However, because he has other issues with crowds and changes in routine one of his therapists recommended fidgets. They are small items he holds in his hand that he can fidget with. Examples are small balls, some of which have flexible tendrils. He had a squishy ball that looked like it had an egg inside, a small wooden puzzle he could move pieces around. Playdough can be helpful.His mom finds these in Fred Meyer, a one stop shopping store. They help him focus. He's also allowed to chew gum. Last year's class had students sit on large exercise balls. He is able to move around on the ball to keep it in place. When he is over stimulated he can walk in the hallway. During the play his mom took him to the hallway so he could walk. He's 11. He can also pace in the classroom in back of other students so they don't also get agitated. Has he been evaluated for sensory issues? With an IEP, there should be a trained person evaluating his behaviour along with education issues. I would bring this up with at the next IEP meeting.
My granddaughter also has an IEP because of ADHD. She is in a regular classroom. Schools have teachers in a seperate room who works with IEP students and their parents. I don't remember the teacher's title. I would talk with that person in your son's school. When my daughter was in elementary school her mom spoke frequently with this person.
Sounds like he isn't getting therapies to teach him skills and that you don't have help learning about what causes these issues and how to manage them. I would try to get involved in providing therapies if the IEP professional would consider them helpful. I don't know his diagnoses so don't know what if any form of therapy would be helpful.
I, too, wonder why the teacher is requiring students to sit still. I also wonder how familiar with managing special needs students. When my granddaughter was in Kinder she wandered and couldn't stay focused even during performances for parents. The teacher arranged for her always have contact with her chair. While other kids sat on the floor for circle time, my grandaughter used her chair to keep her with the group. During one performance, she stood at the end of a line with her knee on the chair while the rest of the kids stood.
Managing in these situation takes creativity. Perhaps the music teacher could assign him a task that allow him to move around some. I've seen music classes in which students clapped their hands.