M.L.
You can use this situation to teach your son a little more about friendships, and to help him sort out what he already knows. This won't be the last time he meets kids like this, wherever you live.
It may be good for him to realize, as young as he is, that he doesn't have to do what his friends and his playmates tell him to do. He doesn't have to listen to what they say. He does not have to be the follower. He can be a leader, or at least independent. He can say no, and he can choose to play somewhere else and with someone else (or alone) without having his friends' approval. He can also learn to use the time-honored line, "I won't do that - my dad wouldn't like it."
You don't have to say, in the time you have left at this location, "No, I don't want you to play with this boy at all," unless of course something serious happens. But you can say, "How did playing with Jack go?" and keep the communication lines open. Poor Jack - he certainly doesn't know very much about how to be a good friend. What you want your son to learn is how to manage the future Jacks he will meet.
Don't worry about the child tracking you down. Almost-third graders don't often do that, except perhaps in movies.