I have 2 responses to this.
First, in the moment when you see what might not be a healthy dynamic, let him work it out himself.
However, that doesn't mean that you shouldn't have conversations about what makes a good relationship. I recently attending a "growing up" seminars for boys this age (age 9-13) with my son. It was AMAZING. With the facilitator, we spent time talking about all kinds of topics. Most of the topics were puberty related; however, there was one section where he had each kid name his best friend and then say why that kid was his best friend. Once there was a list of qualities that made a good friend on the board (fun, easy to talk to, lets me be myself, etc), he had them make a list of qualities that were the opposite of that (makes me feel bad about myself, tries to make me do things I don't want to do, etc). Then, he invited parents to talk one-on-one with his/her son about relationships - what's a good relationship and what's not a good relationship. He encouraged us to talk about friendships first, including peer pressure, and after a few minutes, asked us to talk about how those same good qualities of a friend might apply to relationships as they grow, such as when dating.
This has opened up all kinds of conversations between my son and me. And it gave me the words to talk to him about his friendships - what he likes about friendships and what he doesn't (in general, not naming specific kids). Then, go back to my response #1 and let him decide how to handle his friendships.