If your son is getting punched, he needs to report it to the bus driver, the bus monitor and the principal as SOON as it happens and he gets to school. You can't do anything to help him when he tells you 3 days later.
You can ask the bus driver to assign him a seat close to the front of the bus so that he won't be near these boys, will be within the view of the bus driver so that when these things happen, the driver will be in a better situation to see or hear it.
We had a similar situation with my son on the playground for a few years. We had meetings, etc. nothing helped. Then the boy just stopped. They were in the same class (3 years running, grumble) and last year he just stopped picking on my son. They actually became friends.
Help your son stand up for himself. Teach him a few zingers.
Instead of him saying 'quit it', 'stop it', 'leave me alone', 'i'm telling'
try: - 'so you've said. get over it.'
- 'fine. you're boring me.'
- 'makes it easier to see at night'
- 'so better to see you with.hehehe.'
Give your son permission to defend himself. If the older boy hits or punches him, give your son permission to hit him back one time. That's usually all it will take. Again, my brother had this problem when he got to middle school. bigger, older kid was picking on him for being smart. so he started getting bad grades, keeping to himself, having a bad attitude. Mom found out and said I don't think so, what's going on? So brother finally told her that he'd been getting picked on in the hall by this kid - tripped, back of shoes hit, books knocked out.
All 5 of us looked at him and said one hit. That's all it will take. You are getting picked on because they know that they can and you won't do anything. Brother: But I'll get sent to the principal! Mom: 'so, I will gladly pick you up, and give them a piece of my mind while I'm there'.
A few days later brother comes RUNNING and WHOOPING into the house. 'I DID IT! I DID IT!'. Did what? I stood up! What? ... the kid started picking on him, tripped at his shoes, knocked his books to the floor. brother took a deep breath. turned and shoved the kid against the wall! Looked him in the eye and said 'Do NOT do that again!' .... and then walked away. That kid NEVER bothered my brother again. One time is all it took, and he didn't even have to hit him!