Question: How do you explain divorce to a 7yo?

Going through separation right now. Trying to do this the right way and being civilized with my ex, but explaining to my son why daddy doesn't live in the house anymore is harder than I expected. He keeps asking if it's his fault which BREAKS ME. We tell him it's not but I'm wondering how other dads have handled the conversation. What language worked for you? Any books that helped? He's pretty smart for his age but also just a kid who needs to feel secure.