J.W.
Aww hun you are projecting. He is fine. If you keep doing that he is going to start reacting to your perception. Not sure if that makes sense. It is just as kids grow they change, you cannot attribute that to a new sib, it would have happened anyway.
He is just adapting to the world around him but if you start thinking he is sad he *will* start thinking there is something to be sad about. It is a self fulfilling prophecy.
Just to kind of give background both my son's have sisters two years younger than them, both of them were over three before they started talking, neither of them really cared that their sisters came home though when they were around eight they did tell us they were big mistakes, usually after something they treasured was broken by said sister. I have four kids with two year gaps between genders and nine years between sets.