Are you sure that the wild goose chase on Halloween was purposeful? I recall the same situation happening to one boy I know on Halloween, but the other boys weren't doing it on purpose, they were just poor planners.
You have to understand that kids that age, especially boys, are kind of socially inept, especially when it comes to planning. They get in the moment, think, "Let's go to X's house," and any previous plans they made immediately go out the window. It might mean that your son is less important to them in the social scheme of things, but not that they are purposely avoiding him.
Avoiding their peers is less typical of young males, who tend to just kind of go with the flow. It's usually young females who engage in ostracizing and social drama.
I feel like your son is probably accepted by his friends, he just isn't one of the leaders, so he's more expendable. If he is choosing to stick with these friends, there's nothing you can do, other than encourage him to get involved in outside activities.
I know it hurts as a mom, but I really think you might be reading too much into this. I think these boys are just being typical clueless boys, who easily forget that they told someone else to meet them somewhere, and have a spur-of-the-moment mentality.
I have observed so many fifth-wheel type of boys sitting at middle school lunch tables, and I can promise you that the other boys are not actively avoiding them, and are not mean to them. They are simply the kinds of boys who are followers rather than leaders, and they fade into the background a bit. But that doesn't mean that they won't do very well in high school or later life, or that they are uncomfortable with their place in the social hierarchy.
Middle school is a time of changes. Your son will survive this.