The driver says he yells and won't be seated. Is that accurate?
If so, then it would seem your son's behavior is an issue that needs to be addressed at one level or another.
He is a generally well behaved child. Before the death of his mom, he attended a school for the deaf, at which he stayed during the week. He had a 2 hour bus ride many times (school bus) and longer on a greyhound style bus. We never had a problem then, and I asked. I take very seriously the behavior of my son. He speech and hearing are a major issue, but he understands and is respectful. I am not overbearing, but I do not tolerate or expect less than good behavior.