I Can't Believe I Let This Happen...

Thank you all for the support I got. It really scared me when you suggested my 15 year old might be being sexually abused by my ex's girlfriends 23 year old son. I tried not to think about it.... But it just... Started making sense.... Him randomly started wetting the bed. Acting scared and panicked when ever her son was around. Having a panic attack when her son wanted to hang out. I sat my son down and just asked... "Does he touch you?". My son looked away and said no... I knew right away he was lying. I begged for the truth. That he wasn't in trouble. He starts crying saying he's sorry and it was his fault. How he went in his room when he wasn't supposed to. I wanted to cry... But I had to be strong for him. I asked was it just one time... My son paused and said "No...". I asked if he just touched him or was there other stuff involved. He looked away and said "I didn't want to... I'm sorry". I couldn't anymore. I started crying. My son started begging me not to tell anyone. I told him I need to do what's best for him and he started crying and panicking again. I told my brother to stay here with him and keep him calm. I drove to my ex's place. He opened the door and I see her son. My body boils. I scream about how her son is a pedophile and what our son told me. His gf comes in and asks if it's true. Her kid legit says no and starts smirking and trying not to laugh. I was going to kill him. He looked me in the eyes and smirked. My ex told me to leave. I asked him what about our son. Then he told me this "He doesn't have to come see me anymore if he doesn't want to. I don't need his drama". I can't believe he just dismissed this. And her son is fucking laughing again! I thought about trying to stab him but I just said "You're a poor excuse for a father" and left. My kid locked himself in his room and won't talk ti me. How did I let this happen. I sent him over there for months! I fucked up my kids life and I can never forgive myself for letting this happen.