K.M.
Definately don't say anything. There is a reason why your son wasn't invited and it is most likey superficial and ridiculous. Your son didn't miss out on any thing. If he feels hurt about not being invited let him talk about his feelings and let him know it will happen in life. Not every one he comes across is going to like him and it is their loss.
I'd take it as a clue but not take it personally. I wasn't invited to hundreds of birthday parties growing up and I'm still not. Haha. It's a sucky feeling every now and then but, I have a much better time with my family and TRUE friends anyway.
If your son feels like he missed a good time, then you show him a good time. Let him invite a true friend or two for burgers and arcade or what ever he wants - within reason of course.
This is a great learing lesson for him and you. Not every one meshes together so sweetly all the time. The best you can do is always stay polite and kind no matter what. Who ever can't appreciate that isn't worth thinking about. And, if there is a serious problem with your son and/or you that they have it will eventually come out.
For now, don't give them the satisfaction of feeling hurt over not being invited to a party. Your son has so much to offer and not everyone is worthy of his friendship.