Have you asked your DS what he thinks of this?
There can be a benefit to self-study time. In fact, I just had a successful advocacy for it in my DD9's case - she's in a G/T pull-out for ELA, where among other things she journals every day, then comes back to homeroom briefly where her class was journaling every day, so I requested DD receive permission to use that time to work on homework from other classes instead. I figure, if DD has to be there for 7 hours a day, she should be able to use that time productively.
So, if your son wants to be in the band, but doesn't have anything to learn from the band class (and most importantly, your son's band teacher agrees), wouldn't having that time to practice on his own mean he doesn't have to practice at home? Could he also use that time to work on other school work if he wants? And wouldn't either or both of those activities during band leave him with more free time at home?
Sounds like it could be a pretty good deal to me.
