Never ask for advice unless you want it. If your son is loud and intimidating, you should find alternate transportation rather than complain that your child should receive a "special" bus driver. You are basically blaming the bus driver for your child's behavior. There is no reason your son can't be quiet on a bus. One loud kid tends to start a tsunami of loud kids.
First, I drove a school bus for 2 years so I will give you my perspective.
Bus drivers make extremely low pay. After taxes and insurance I took home about 600.00 per month. That being the case, most of the people who drive buses are either retired or housewives who do it for the benefits. I found that a some drivers didn't have the patience for the kids but most loved the kids, myself included. That being said, one of the most important rules on the bus is to stay in your seat and to talk quietly. The driver needs to concentrate and you have to be able to hear your radio. With elementary children if one gets loud, they all get loud. If one is up walking around before you know it they're all hoping from seat to seat. It can get very hectic when you have 50 kids behind you jumping around and yelling. Also, from my experience, the school does not inform the bus drivers of special needs for privacy reasons, unless its medical such as asthma or diabetes. While it doesn't surprise me the bus driver is impatient, on the other hand the driver has to maintain control of those kids. Imagine if the bus was in an accident or hit another child walking home because of the distractions. Maybe your son would be better off on a special needs bus, there are far less children and the drivers are better equipted to deal with what you are describing or try talking to your son and the transportation supervisor. Good Luck