C.P.
Hi Diana,
I've really noticed this kind of behavior with GIRLS! I have a 3.5 year old son, and we've seen the same thing. In fact, some of these girls are downright nasty and say things like "no boys" and "go away, we don't want to play with you..."
Is this normal? I don't know. But when I see kids being excluded from play, I tell my son that so-and-so needs someone to play with and ask him to go over and play, and he does! So what's with other parents? Do they think that this kind of behavior is okay? I think all children should learn manners and learn that it is not nice to exclude any kid like that, especially when they are at your house to play! that's nuts. Where were these girls' parents that they allowed this to happen and left your son playing alone like that?
Now, all of this said, I would probably just tell your son that those girls don't know how to behave nicely and find him some other playmates who parents are more diligent about teaching proper behavior.