Speaking only for myself...I would make my position and displeasure known personally to the bus driver.
I would make sure that he/she knew that I was aware of his/her unacceptable behavior towards my son and any further distress or disharmony directed towards my son would be met with high levels of displeasure from me.
I would also make it known to him/her that my displeasure takes many forms...none of which would be pleasant for the bus driver.
I know that many here will say, "Oooh you can't doooo that."
Tough. When my kids are harrased or bothered I respond quickly and severely. Besides, we're talking about a child with enough problems on his plate without having to deal with a boob like this. If this bus driver can't act like an adult then it's high time someone put the fear of G-d into him/her.
Again, I speak only for myself and do not suggest you respond in this manner unless you have experience doing so and are good at it.
I am with you on this one. Don't take no for an answer.
I drove my kids to and from school every day until my oldest daughter was old enough to drive and take herself to high school. Her first day of school the bus driver left her off at the wrong gate at the elementary school and she was lost just walking around. After that I took my three kids to school and picked them up.
In Yucaipa one time a bus driver got mad, quit in the middle of the route, told the kids to all get off the bus and took it back to the bus garage, got in the car, and left. My best friend's daughter was on that bus and it took the school over an hour to find the kids after they didn't come home.
Call me an overprotective Mom but I didn't trust my kids to bus drivers. I was fortunate that I was able to accompany my kids on field trips and drive them to and from school. Had a bus driver one time on a field trip that didn't know how to drive a manual shift and one of the band kid's had to help the guy shift -- was sure I wasn't going to make it home alive as the guy kept killing the engine at stop lights.