Hi. When I read your post I felt so empathetic to your pain. Unfortunately I was in a similar situation a few months ago. My 16yo son has had ongoing issues with bipolar depression (w/& wo suicidal ideation) for years, so acting out was not unusual. One day he came home from his dad's and was absolutely mentally abusive to me--I mean a whole new level. Hours of acting out. Then he said it.
Starting when he was almost 5, the 15 yo son of his dad's girlfriend molested him & my older son, then 9, repeatedly & together over the period of a few years, also making my sons do sexual acts with each other. The molester spent time in my ex's home after the breakup w/girlfriend. The day he told me was the first time he'd seen that woman in years. I was also completely shocked, pissed off, & confused on where to begin. His dad is no help. My younger son wanted therapy, but the first professional he spoke with turned him away but called for an investigation by DCFS (child services). That shut him back down & tore the family apart. He has started with a new therapist, but is still like Jeckyl & Hyde in how he treats me when we used to be close, and he's rarely made it through a school day.
My older son, now 21 still does not want to talk about it, but has disclosed some of what happened. I can hear him shouting & crying about it while pacing the house in the middle of the night. He has schizoaffective disorder & the thoughts bombard him when he's psychotic. I can only encourage him.
I wish you & your son the best as the healing can begin.