I agree, perhaps I am more concerned with how he holds a grudge, takes things very personally and some underlying anger, I don't see this in any of his peers. I know things could be much worse, but I feel this is serious enough that I would like to help him out.
Thanks
Perhaps you are over reacting and maybe it is worse when you are there, if you had left him at the party he would have joined in with the other kids, take a step back and let him get on with it, it was a normal feeling for him to feel left out when his friend wanted to be with the other children, we all want our children to be happy all the time , Some things he has to fix by him self , let go a bit and see if he does okay. Good Luck