My 41-year-old Son Has Just Been Diagnosed With PTSD...a Result

My 41-year-old son has just been diagnosed with PTSD...a result of childhood sexual abuse and later abuse by an adult. He is in a good behavioral health hospital, getting treatment. He has a 17-year-old son who knows his Dad is in the hospital, but has no idea where or why. My son has asked me to talk to my grandson about this, and I am very willing to do so. How much detail should I give my grandson about his Dad and the reason for this diagnosis? YYYY@YYYY