Your son, at 14, is old enough to talk with him about it. He may be happier on his own. Socializing is exhausting for me and always has been. Given a choice as a child, I would always have preferred to be left alone, and resented my parents’ attempts to force me to be more social. You could mention to your son that you sometimes worry about him feeling left out when you plan social events for his sister. He may not be bothered by it at all. You might ask him if there are things he would really like to do. I hated being forced to deal with humans, but I was over the moon to be taken someplace where I could interact with animals, or to someplace natural, where I could roam on a beach or hike to a waterfall. If you include your son in the discussion, I think he can guide you better than anyone here can. But what I would NOT do is feel guilty for treating your children as individuals and not forcing one to be like the other.