S.W.
I agree with asking to speak to the band director. Find out why he was not picked for the higher band. Knowing why he wasnt chosen may make it easier for him to deal with. Disappointment is never easy and at his age one thing can seem like the world, but it is a part of life. If he finds out why it may help him in the future.
One reason could be there are a lot of students that play his instrument and with some extra lessons he could change to a different instrument that would allow him to play in the higher band.
As far as having different classes then his friends he will be ok. My daughter has few classes with her friends. She is in band and many of her friends are in choir so their scheduals are different. Actually some of her best friends she has no classes with. If they are true friends they will stay friends and he will also meet new friends. He needs to learn there are two sides to everything.