Oh Rusty, what a tough time for your son.
Bullies push other people around because they CAN!
They won't stop till someone makes them stop.
It's just as bad with girls, but they use shunning, teasing and taunting more than their fists, (but not always.)
Yes, tell your kid it's not his fault. Yes, tell the teacher. Schools
are obligated to put a stop to this during school time, and that includes the bus ride too. Hitting cannot be tolerated. I our school system, hitting is handled with conference, then suspension with counseling, then expulsion or placement in alternative ed.
I hope you find the the resources that can help you to empower your son to become more "bully proof". Hope you find the tools for that in the posted links.