First off...I am so sorry this is happening to you. I can't even imagine. I wanted to give just a word of caution, too, regarding talking to the other kids on the bus. You as a parent are not allowed to ask questions (of this nature) to children on school grounds. I do think it is a good idea to get the other kids' perspectives, but it would be best to talk to them with their parents present off of the school property. Talk to the parents ahead of time to let them know what you would like to discuss so they won't be caught off guard. Good luck. I hope it works out for you.
Google 'XX Crimes Code and Criminal Law, Wiretapping' (XX = your state's initials) for info re legality of using recording devices (RE: 'micro' recording device comment above).
Better inform yourself of your states laws about tape recording (ref micro tape recorder comment -I am assuming from the context it means do it surreptitiously) in many states it is a FELONY offense to do so as a layperson (as opposed to law enforcement or govt with valid reason etc), without asking consent of the party you are taping.
That is one peice of advice on these boards that is very very very very iffy depending on the laws in your state.
Tell your son to keep a journal too. I have found this to be helpful when my nephew was in school. Write the time, date and the incidents as they happen. This way when someone (administration) asks what happened you have your who, where, when, etc. I even got my mom doing it and she's a manager. If it keeps up there are always micro tape recorders. I hope your child is okay. That sucks and I commend you for keeping your cool.
My friend went through something similar to this. Her sons bus driver got mad at the kids and slammed on the brakes so hard that the kids flew foward and hit the seats and the bus driver left skid marks. Needless to say my friend wasn't happy. She immediately called the bus garage and they deined all, so she called the cops, in the meantime the bus tape from that coveniently disappered. So I believe that they can be sneaking. Stand your ground and fight back. Document everything, keep a pad of paper by your phone and anything you question that they say get in wiriting. In my friends case, the bus driver still kept her job, and had to apologize but that's it. The bus garage wanted to protect their butt and the principals wanted to protect there's in the end the bus driver got away with it. A week later the bus driver put the bus in the ditch but was only giving a ticket for faliure to control, but she's still driving the bus!!
Perhaps you can address your state department of education, google them and search for COMPLAINTS. If ever there was a 'good' complaint, sounds like you have a very legitimate one that should not go unnoticed by the heads of your state, whose job it is to look after the welfare of children.