I haven't heard of this problem yet myself, but my first thought would be to contact the school bus company and/or the building's principal. At our schools, the principals have always been very helpful working out bus issues. It could be that this bus driver has invented this policy on his or her own, perhaps due to bus overcrowding. Hopefully you'll find a resolution there. It seems to me that the kids should be able to bring all their school supplies (in this case, his horn) on the bus.
If it turns out that the policy must stand, maybe you can brainstorm with your band director to figure out a plan B. Can your child play a school instrument at school, and keep the other at home for practicing? Can you figure out a carpool schedule with other bus-riding band students?
Good luck! Let us know how it works out!