Tough one, LaBoo.
I have heard my exN described as an "arrested adolescent" (although I would call him younger).... and I am not sure what age, exactly, if there is one, that kids are supposed to start acting "grown up". Does your son live with you still? Is this behavior only with you and his family or is it everywhere?
My kids are adolescents, and trademark behavoir for adolescents is selfish by definition....at least at home. Out in the world, everyone thinks they are wonderful, which I know to be a truer version of who they really are than how they behave with parents (they won't live with me forever).
That said, their dad is a textbook NPD, so I do worry about this.
They have seen Dad win in court, control our lives because of that, and I have concerns that they will see this as benificial behavoir. However, they also see that he is angry most of the time, they do not feel free to have "real" heartfelt discussions with him (they just don't do it) and do not feel he has their best interest at heart.
I have a friend, though, whose oldest child is angry, abusive and unhappy, just like her ex. She is at a loss, and now can only protect herself and her other children from his anger.
So, lots of help I am, but would love to see some responses. Good question!