M.O.
He sounds absolutely, completely normal and fine. He just sounds like he has great receptive language (i.e., comprehension) and is waiting until he's more "ready" to try vocalizing words. Some babies are like that -- it's fine. Could even be a lifelong personality trait -- he could wind up being the type of person who listens and takes everything in before jumping headlong into things. But he sounds absolutely on target for his age.