I think you can handle this in a few ways

1. Speak to your 504 coordinator first to make sure that you are both on the same page on how to handle this situation

2. Private correction could be a simple as making eye contact from the podium, a special hand sign from the podium to re direct behavior (that the student understands that the sign means to reel it in) a sticky note on a music stand with playing corrections or behavior corrections

3. Correcting the entire section is fine - more often than not a student with this particular accommodation does not do well with being singled out and when that happens it often results in worsening behaviors

4. If the student is doing fine in band - document - when you give correction as a section notate if the student does well with that and how often this occurs - if the student does not need that accommodation in band you can document and ask your 504 coordinator if it can be removed from band - 504 plans are fluid documents that can be changed as needed based on student need - AGAIN speaking as a former 504 coordinator - ALWAYS consult your 504 coordinator to work together to make this work for your classroom and the student - it can be done