H.W.
ETA: I want to add one thing here, which flies in the face of some wisdom: one of the ways parents really mess things up is putting *pressure*on our kids to 'stand up' for themselves. This can be really hard for some kids, who are afraid of getting other kids in trouble or that their friends might not like them. Instead of telling my kid to 'stand up' for himself, I ask him "how do you think you want to handle that the next time?" Because there often is a next time. I don't tell him how to deal with stuff--- he's had one classmate who is way to physical-- Kiddo feels the kid 'doesn't listen' so he wrote the boy a card to say that "we can still be friends if you will be nice".... that's his level of confrontation. He will defend himself in the moment, but is still a bit anxious about friendships. Giving him encouragement to address the situation in his own way is better than saying "stand up for yourself" which many kids may not know how to do on their own, esp sensitive ones. This is lifelong process, learning to say "no", and some kids get it right away, others need time to mature and let their perspectives mature as well. It's hard to watch, but necessary that we don't fight all of their battles for them-- and that we let them figure out *how* they want to fight those battles.
Here's the deal-- you will have to decide where your 'line' is. I say this as a mom who had similar issues with my son last year, seven and in first grade then.
Some of this is actually a great opportunity to practice assertiveness. Saying a powerful "no", standing with hands out, forward, when someone approaches in an an unwanted way. We practiced and practiced and practiced. Your son has to figure out what he will and won't tolerate.
When it got to the point that I didn't feel my son was being kept safe, I approached the school and told them I was going to remove my son from the school until they came up with a safety plan for him. I couldn't force the school to 'do anything' to the boy who was deliberately targeting my son, but I could control my side of things. I went after drop-off one morning and told the office staff I was taking him home because he was being bullied and that, as a parent with a conscience, I couldn't leave him at school when he was so terrorized and humiliated by this kid that he wouldn't tell the teacher when he was being punched (in the crotch, no less).
I didn't approach them with bravado, I should say. I went back to the school nearly in tears because I felt horrible about leaving him there. That morning I spoke with the principal and assistant principal, who then contacted the counselor. By seven pm. there was a safety plan in my email in box. The boys were never to be left alone during transitions, never left unsupervised, placed physically away from each other, and we worked on identifiying who our son's 'safe person to tell' at school was, which happened to be someone other than the teacher, who was a bit brisk with him.
This resolved in time. The boy and my son now play every so often on the playground. It was unfortunate it had happened-- I found out later that this kids' family was also going through a terrible divorce. I try to remember this when I see this kid--- he can be mean, yes, and he has a hard life. Having him removed from the class or the school would have only made it that much worse. Having a safety plan in place allowed the boy to have the consistency he needed and my son to have the safety he deserved.
There will always be kids like this, so start now in helping when you can but not stepping in too much. So, talk to the other adults in his life about keeping the boys on opposite ends of the room, helping them to redirect the troubled boy when he's approaching your kid... I did find that talking to the parents really didn't help at all and was very frustrating. Talking to the other adults in charge in a non-judgmental way, just "this is what I am noticing, could you help...?" Be as solution-oriented as possible. That the best advice I can give you. Good luck!