Sometimes exclusion is intentional and malicious. That is a very real, very painful experience and requires a unique approach.
However, before accepting that conclusion, I would suggest something else first. When your child feels excluded, I would start by looking carefully at the activities in which he or she is involved, and see if there have been any changes lately.
Over the years, I've noticed something that has a profound impact on the social lives of middle school students, but is often minimized or completely ignored by adults. In my experience, this is to middle school social experience what Newton's laws are to physics, and is at the heart of many, if not most, social struggles.
The groups to which students are assigned, or the groups in which they voluntarily participate, have an enormous impact on their social activities. Current activities will almost always trump pre-existing friendships. Teens will generally form friendships with those to whom they have the most proximity. Let me explain and give a few examples.
A few years ago, one of my children lamented that she didn't have any classes with her friends. I somewhat naively told her that wasn't a big deal. She could see them at lunch, snack, between classes, and so on. It wasn't as if class time provided a lot of great opportunity for socializing anyway. Despite my clearly reasoned explanation, she remained devastated. And, she was right. That year, her social relationships and friendships definitely shifted--largely as a result of this structural change.
I've seen this happen over the years more times than I can say. Two children are best friends. One of them tries out for the school play, the lacrosse team or joins the robotics club. The other doesn't. Very quickly, the relationship starts fading. One of the children feels hurt and left out; her parents worry about exclusion.
Where I went wrong with my daughter--and where I think many other parents do the same thing--is in realizing how definitive these groups are for adolescents. My best friend the world is not a teacher. He does not live terribly close to me. In fact, we don't see each other much at all unless we make it happen. But that's the thing--we can make it happen. I can drive over to his house. We can meet for lunch. We can invite his family over for supper. External barriers of schedule, time, and place don't mean much to adults because we have the means to work around them.
A young adolescent who can't drive, have limited funds, and is dependent on an adult for everything does not have these options. So, the people with whom they spend their days are the people with whom they will develop bonds.
Beyond that, it's a normal human response to develop relationships based on shared experience and proximity. But it's especially potent for teens who have no other options.
There is another aspect that is important to understand, also not something I think most adults quite understand. I define myself in many ways. I'm a husband, father, teacher, director, author, singer, Mormon, son, brother, on and on. All of these are aspects of my identity.
Most adolescents I've known are far more concrete. They are far less able to acknowledge multiple dimensions in their identities. They are more likely to say, "I am an athlete," or "I am an actress," or "I am an artist."
Because so much of their lives revolve around trying to build and maintain an acceptable identity, they cling to these somewhat narrow visions of themselves with great force. Their identity makes them see themselves as only one thing, and those who are not part of that same thing may seem far enough outside of their experience that they no longer see that they have much in common.
This is either caused or magnified or both (not sure) by the tendency most young teens have of being rather exclusive in their friendships. I've noticed that many girls have a best friend relationship that almost echoes a dating relationship in later life in terms of the exclusivity of it. Boys do it as well, but in my experience, they seem to be more oriented to small groups than a specific best friend.
The good news is that these categories are not terribly durable. They are keenly and fiercely felt, but not always deeply rooted--they tend to change as circumstances change. I've seen kids who became new best friends because they were both leads in the fall play. They were fully in the drama tribe, that was their identity and their social marker, they were sworn to eternal BFF-ness, had turned their backs on their former friends--and then the next play came. The casting meant that they didn't have parts that put them together. One started swimming and re-connected with all her old friends, the other started getting involved in writer's group or something.
The other good news is that, over time, these experiences and new friendships tend to mellow the personality a bit, providing a more balanced perspective: an openness to new experiences and people.
The bad news is that while this process is happening, it is very easy for a child to experience exclusion. If, for example, your child is the one who is doing hockey and everyone else is swimming, or you are in travel soccer and your child's peers are all in the play, then your child can quickly become excluded.
Everything becomes focused on the activity shared in common by the majority of the group: inside jokes, lunch conversation, Instagram pictures, on and on.
Often, this exclusion is not malicious or intentional. But that doesn't make it any less painful.
Still, if you understand the cause, it can be addressed. Arranging activities with friends your child wants to connect with is helpful. One-off things are great, but if you can find a way to have them do something together on a regular basis, that is probably going to be more effective. This might not always be the case, but it is the place I would start. Sort of the equivalent of taking some ibuprofen for a headache before you get an MRI.
None of this is perfectly predictable. When you deal with adolescents you take every possible human quirk and variation and blow it up by several orders of magnitude. You add hormones and insecurity and lack of clear, rational thinking--things get crazy quickly.
One other thought: sometimes changing social situations can be beneficial. It's very painful to have your friends go a different way. It happened with one of our children. A friend of many years suddenly just went off a different direction. There was no malice in it, but our child was still hurt. Honestly, though, that friendship was a bit one-sided. The fact that it changed gave this child a chance to find a new friend, one who was just as invested in the friendship and seemed to care just as much. In fact, this friend seemed to be actively looking for new friends as well after experiencing some tectonic social change as well.
As always, empathy and love will help. Patience is a must. Your child has no life-experience to draw on, nothing to reassure him that this will ultimately be okay. So, that is an important--even critical--thing for you to bring.