Bullying is a serious problem and as you may have seen on the news some children end up committing suicide over it. If your child is being bullied you need to first go to the school for help. They need to know all the details, who the bully is and what exactly is happening. If the school does not do anything then take the situation to the authorities since bullying is now illegal in most states.
I agree with first person who commented. Also, insist your child to ride directly behind driver so he/she is in sight. You can also talk to principal of the "bully" and so many complaints and they will be suspended from the bus.
Give his bullies the belt! Teach him how to fight! Or you could stop dressing him in nerdy clothes, show him how to talk normally, and stop making his hair completely neat and tidy. Other than that, talking to the principal really wont work because they will assume that he has told someone and as we all know, SNITCHES GET STICHES!
It is the schools responsibility to make sure that your child is safe and secure while under their supervision. I would hold them accountable and include local law enforcement if necessary. The problem with bullying is that it can carry on outside of the school setting. You should consider placing your child in a martial arts class.