Sounds like you are doing all the right things.

The sad fact of life is that kids who are bullied often go on to become bullies themselves. It's also completely normal that your son is simply too young to be able to explain WHY he's doing this. It could be that this is how he has learned that older boys behave towards younger boys. It could be a case of this is what he's doing to "look tough" in order to prevent becoming a target again.

Another excellent author on the matter is Barbara Coloroso. I have one of her books here "the bully, the bullied and the bystander". I've never sat down to read it in detail (saving it for when I have kids rather than just teach them) but I did get to attend a professional development course run by her and it was fantastic. A book well worth getting hold of if you can - it's written for both parents and teachers and deals with just how the cycle can be broken.

BW