You may also wish to document each incident. Work with your child to get a clear idea of background setting or circumstances, and also who said (or did) what. Document in brief bullet points, keeping each action in chronological order.

Bullying has been discussed on several threads. Here is one post.

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At what point do you tell your child to fight back?
You may wish to tell your child to say in a loud, firm voice, "STOP. Don't push/hit/take that from me" each time the child pushes/hits/takes things from him. Benefits may include:
1) Your child appropriately stands up for himself.
2) Bully receives direct instruction to stop.
3) Teacher may become aware of incidents, if not already aware.
4) Other children may become aware of incidents, if not already aware, thereby reducing bully's influence.