How do I explain why bad things happen to my 6 year old?

My son asked me yesterday why his friend's dad got cancer. I froze. He's been asking a lot of hard questions lately about suffering and pain. I want to point him to God's plan but I don't want to sound like I'm dismissing his real confusion. How do you guys handle these conversations with your kids? I've been praying about it all week but I feel like I'm missing something.