Just found CP on my Bf’s phone

I don’t even fucking know where to begin. I’m in a total state of shock and disgust right now. He has TWO little sisters (3 & 5 yrs old) and now I worry for their safety. I’m half tempted to tell his mother of the CP. I WILL be taking evidence and going to the police. He has an issue with R/dirtyr4r and got the CP from someone he met on that subreddit, who then sent it to him on the teleguard app. I really want to beat him within an inch of his life over this right now