My poor DS is very confused and says friends shouldn't hurt each other.
That is really sad to read Tan. My DS is the same age as yours.

Can you make him understand the statement he made? As in, "that's right, friends don't hurt each, that means he is not your friend".

It is such a worry that the boy who he thinks is his friend sounds like a kid who needs to take advantage of kids who may not understand, to make himself stand out from the crowd. How do you get into the headspace of why this other kid is doing these things???

Perhaps talk his teacher and let them know what has been going on. Mention it to the boys Mother as well. She needs to be made aware.
Hope everything works out ok