Activate the imagination via age appropriate (not traumatizing) stories based on reality. It's like pseudo-life experience. Activates empathy while exploring the opposing perspective. Children are imaginative, particularly gifted children (high Openness in Big 5).
Son is empathizing while seeing one side, has limited experiences...so get him to empathize with alternative perspectives via pseudo-experiences: stories that place you in their shoes.
Encourage him to think for himself. Teach him to master his thinking/logic skills, don't try to control his views. It won't work anyway.
Why is important. "What" and "because I said so" is ineffective on profoundly curious minds. He has to understand reasons, it's not enough for you to. Same goes for understanding the appropriateness of time and place for discussing controversial topics.
