I think the first time DD1 really showed sadness was when she was 3.5 and she cried while watching a movie where one of the characters was sad. I don't think she really gets a lot of sadness in her life (anger definitely) but I was actually really pleased that she showed empathetic sadness for someone else. I'm sure your DS will develop that emotion as it seems the basics are there in tthe recognition of sad and the other emotions. i would be concerned if neither of those were present.