I think my friend's parents are selfish and I going to tell him my opinions on this.
Is this something you are going to tell him all at once or over time?
I'm going to tell him carefully as I heard that way he might not hate me.
It is good to be honest with friends but you have to be careful when talking about their family. It might make them angry since it is the people that he cares about the most.
I just want him to realize how nice he is to them.
I think your friend is lucky to have you because you recognize his kindness, but it's not always a good idea to involve yourself in the affairs of a family that isn't your own. It's unfortunate that his parents are selfish, but he might not take well to your criticisms and this could put your friendship at risk.