Young child learned about George Floyd - advice?

I could really use some advice. I'm putting feelers out in multiple places, but wondered if any of you might have ideas. My adopted little boy (4 years old and black) has been having nightmares. This morning he finally told me why after gentle prompting about it all week. Another young child at his preschool told him a "for-real" story about a policeman killing someone. The police put his knee on a man's neck until the man couldn't breath and then he died. I asked him what else he had learned from the child and he told me that was all. We had a short conversation about it - essentially how sad it is when someone dies, that what the police did wasn't okay, and that doesn't normally happen, and it's important to know that he's safe. Then the cat jumped on the bed and his attention switched over. I think I probably didn't do enough/do this right, but I was so stunned and trying so hard to not have a big reaction so we could be in his moment, not mine, that I did what I could in the moment. So... now what? He's only 4. He didn't tell me it was a black man who died, but I'm worried that was part of the story and he just hasn't said it yet. I'm not sure the best way to help him through this one. We have started the awareness of racism talks in developmentally appropriate ways, and this story is not developmentally appropriate, yet here it is. I'm not sure how to help him process it so it doesn't cause more nightmares. I'm not sure if I need to tell the school so they can think about how to address it in the classroom - I'm guessing he's not the only kid who heard the story. Any/all advice is very welcome.