Does your school have a vice principal? If not, then just go to the principal.
Do you have the names of these kids? Get the names. Does your child carry a cell phone to school with the ability to record the boy on the bus? The reason I ask is that this was happening to my son when he was in 8th grade. The neighbor boy who has a head shorter and a scrawny fellow decided to make him "rep" using my son. He verbally bullied him on the bus every afternoon in front of his friends and they all laughed. My son finally came to me when the kid told him that he was going to beat him up. He was so upset - he KNEW that HE would be the one in trouble because my son is built like an Adonis with strong muscles and he would only look like the bully. Of course, if all this kids' friends joined in, that could be bad...
Anyway, my son's friend recorded what was going on twice. I went to the vice-principal and talked to her, gave the names, and told her that this was the neighbor's kid and I did NOT want to deal with the mother. It was the school's job. I told her that there was a video of two of the bus rides and asked her to watch it. She called everyone separately into her office, did her sleuthing, and then called the neighbor kid in and let him have it. She moved him to the front of the bus, told him if he so much as LOOKED at my son again, he'd be in big trouble, and then she called his mother on the phone in front of him.
We never had one trouble after that. Not one.
She SHOULD have brought this kid to my front door and had him apologize to my son and to me. She should have reached out. She didn't. We don't know each other and we never will because she couldn't bother to make things right.
My son went on to high school while this kid finished up middle school. (Different buses.) However, my son noticed that within a month of school starting, that kid was walking home every day instead of riding the bus. My guess is that he started trying to make his "rep" with someone else.
So you need to start taking names and making your presence known. It is MUCH easier to do this with 2nd graders. This neighbor kid did not know that it was me who ratted him out. He was loud enough that half the bus heard his commotion every day. He probably thought that just because the bus driver doesn't speak English that he wouldn't get in trouble. You can go to recess and watch stuff yourself. Tell your son not to come talk to you so that the kids don't know you are his mother. Then you have an idea of what is going on.
If you don't nip this in the bud now, he'll be a punching bag in a few years. Just do it.
Dawn