TL;DR It's normal, work on mitigation tools, boost confidence.
Bonus at the start of the wall of text
I'm not sure I can achieve the same with my limited skillset in this area, and out on the street.
NON-SENSE You may not have the formal education or the years of experience mediating, but you do have years of being a parent to your child under your belt. You will do fine.
Should I confront the children in the street? Should I start the conversation immediately when the victim is still (almost) crying? Or maybe the next day? But then I may not get the same group of kids together. Should I pay more attention what goes on and intervene before tears?
If you were not asked to step in don't. You may try limiting the time played to the time it takes to normally get to the problem. For example, if they place ice for 2 hours and then it goes south, you may just try limiting to playtime to 1.5 hours. But remember the quote from Dory "Well, you can't never let anything happen to him. Then nothing would ever happen to him."
I am going to write this, but I know it's going to get a few negative votes or reads, but hear me out.
First, when I read the word "bully" I always advise taking a step back and really thinking about what is going on. A lot of current cultures use the label "bully" for everything from "I don't like the color of your shirt" to hanging someone off the balcony of a third-floor apartment to scare them. They're obviously not the same thing, but then they all get wrapped up in a "Zero Tolerance Policy" and treated the same. So first and foremost take care with the "bully" label.
Second, and this can be very hard to do, remember that kids are not adults, and trying to get them to live to our standards and rules is never going to work. As they get older, you can move them into more adult behaviors but when they are young, they have social structures and group dynamics that can be very different from what we would accept as an adult.
Finally, our goal as parents and adults should be to keep them safe and teach them how to react when we are not there to keep them safe.
Now with all that out of the way, we need to address three things.
This is totally normal behavior for kids. That doesn't mean that you ignore it and move on, but I want to reassure you that every parent has had to deal with this, as had every kid. It is how we as humans and social creatures, learn social behaviors. Based on what you describe I don't think you need to worry about your child becoming a bully, or even that he is being bullied every week. Again, that doesn't mean ignoring the issue. But if your child is happy to go out and play when they come calling, then generally it's safe to give the group room to work it out. You said, "that's when he came home angry". That is a sign that your child has the tools he needs.
You are never gonna be able to protect your child from every negative everything, best you can do is give him the tools he needs to respond in a manner you approve of. Coming home angry is a perfectly fine response.
You have to address this problem with your son. You are not the parent of the other kids, and while if you saw something really bad happening it would be time to step in, this kind of this is not on that level. You stepping into the children's play world would likely just make things worse. Again, you have to make a call, and that can be rough. But what you explained in your post does not seem like the right time to step in. What you can do is bolster the tools your son has to deal with this problem. "What do you do if you get angry?" "What do you do if you think someone is doing something bad?" "How can you keep things fun if you think others are being mean?" Ask your son those questions and empower him in resolving the issues. At the same time review what kinds of things are mean. Ask questions like "How would you feel if I did that to you?" "What would you think if they were picking on you instead of Billy?" Again, you're not trying to react to a specific incident, but instead trying to boost your son's ability and freedom to react in those situations. Talk about good reactions and bad reactions. I won't spell those out, you know what is acceptable to you and what isn't.
You do not want to make your son a "poor victim" that needs protecting. You want to make him feel secure in handling the situation himself, even if the only way he can handle it is to get an adult.
That means that you need to support your son in his reactions. If he went to get an adult, praise him for that. If he came home angry, then praise him for that. If he made the bully leave, praise that. Always take your son's side, at least at first. Once things calm down you can talk about how things could be better. This can be tricky. Let's use an "over the top" example. Jack tells Sally that she smells funny and Sally starts to cry and really upset. So your son handles this by throwing toys at Jack and telling him to leave cause he is a "meany head".Obviously, this is not a good behavior that you want to promote. But in the moment, you pop in and tell Jack he better get home. You check and make sure Sally is ok, and ask her to go home. Then you site down with your son and, this will be hard, but you explain that it was good he stood up for Sally, but that the way he chose to do it was not great. You find a way, to seperate the good act of standing up for Sally, from the bad act of throwing toys. And once you have explained that the best you can, you stick with the rules. If the punishment for throwing toys is that you don't get to play with them anymore then enact that, while still enforcing that standing up for Sally was a good thing.
Let's look at a "good" example. Jack tells Sally that she smells funny and Sally starts to cry and is really upset. Your son stands up and tells Jack to leave and calls you over, again insisting that Jack leaves. This time when you pop-in, you back your son up much the same way. Telling Jack to go home. Check on Sally, and asking her to go home. This time you prise your son for handleing the situation. How calling you was the right thing to do, and asking Jack to leave was a good job.
In both examples, you're making teaching the right/better way to handle the issue, and giving your son tools he will need handle people later in life. In the "bad example" you also corrected the means by which he handled the situation while still trying to teach that it does need to be handled.
If your son is being directly bullied/picked on. Your best course of action is to boost their confidence. This is very tricky. IMO you need to teach how to accept critisisms and how to accept them selves. Again, this is very hard. But humans pick on other humans for differences, and the ones we end up most self-aware of are the onces we can't change. "How's the weather up there" to tall people etc. You need to find a way to teach that those negative comments need not have a huge impact. The important thing is that if your child is the one being picked on, even some times, make sure to take time to boost their confidence. That good advice regaurdless, but especially important if they are being picked on.