When he calls everything else daddy & wa wa he is generalizing. It is typical in speech development. He cannot say tractor but he sees daddy on it so he calls it daddy.
Daycares do not provide the best environment for language aquisition. Many times the behavioral issues inadvertantly overshadow conversations that should occur, therefore language usage suffers. So with that I would not be concerned that he isn't expressing himself at daycare as much.
What you see at home is most importatnt because he has your attention and isn't sharing it with 10 other kids. It sounds to me as if you child is doing very well. Keep up the great work mom!