If he drops physics and later wants to do STEM, he can take it at college. My son did AP Physics as a senior and got a 5 but still elected to take physics freshman year, and so did many others: students often like to take the whole sequence of classes for the major at the college they are attending, and I think reducing stress is another factor.
If your son wants to focus on music, it can really good to pare down other things, especially if he has already satisfied a requirement for, in this instance, science. The volunteering may not impact his time outside of school. If it doesn’t, then it is a good way to mature a little, and even gain poise in public.
There are cases where a scholarship is available to the #1 and #2 in the class, for instance, at our state university. Hopefully that is not relevant here. The focus on GPA and rank is, in my opinion, destructive.
Is he better at or more partial to math or English? I would even look at those classes. Unless he loves the academics, and I forget now if academics are also a priority.
You can write a note to colleges to enclose with the guidance counselor letter, or the gc can write, explaining these choices in the context of focus on music. Colleges don’t mind, and even show more interest, in a kid who is committed; and of course conservatories don’t mind at all.