L.A.
At this age, your son needs to start speaking up for himself..
When kingpin says ""come on, let's go inside without him" " don't anybody pick him on your team" " lets not go to the pool with him"
Your son needs to say, "Hey guys, that is not fair." Or "Hey, You all can play with whoever you want." "You do not have to do what he tells you."
Until your son proves he will not be pushed out, excluded, bullied.. These other "followers" are going to continue to follow this kingpin around.
They fear that guy will turn on them, when in reality "Kingpin" is afraid of being excluded.
Work with your son, but try not to interfere when others are around unless there is a physical attack.
I will admit in our neighborhood with all of the girls, if we overheard this type of behaviors, we would sometimes, say, "Hey ladies, those do not sound like friendly voices.."
Our daughter always did well when we could read about a situation and then talk about it. Go to the Book store or the library and see if you can find some books, that speak about Bullying and how your son can handle these situations.
Here is one book that really helped in lots of situations. check it out.
http://www.amazon.com/How-Rude-Teenagers-Behavior-Grossin...
Even though the title says teenagers, I purchased this book when our daughter was in 4th grade.