What to do when your teenager is self harming

My son's ex girlfriend told me yesterday that he's been hurting himself. I was heartbroken to hear this and I brought her in the house so we could both talk to him when he got home. He got home and I asked if I could see his legs and he said no and sees his ex and gets angry at her and asks why she would tell me something so personal and she said she still loved him and I told him we both love him and don't want him too end his life. He got mad and said "She doesn't live me. She just loves that I was too stupid to listen to anybody who told me what a slut she was " I was surprised because I never taught him to call girls sluts and the girl had tears in her eyes and my son said the reason they broke up is because when I let her and his best friend spend the night for his 16th birthday he admitted to his friend stuck them liquor and he got drunk and went to sleep and his girlfriend and his best friend had sex IN MY BEDROOM! She looked ashamed and honestly that would have been great information to know before I thought it was a good idea to bring her to this. He told her "You cheated on me with my best friend when I was at my lowest. Because you're a heartless slut." And then said to me "Mom get this WHORE out of my fucking face." He yelled the word whore in her face and she's just crying.

He went in his room and I told her it was best she leave. She was crying and asked said she's sorry and doesn't know why she keeps hurting people the way she does and wonders if my son will ever forgive her and I told her hopefully one day he will understand that her telling me about him hurting himself was out of concern for his well-being. And that was that with her. I tried to talk to him but he was just crying. I talked to him about seeing a therapist but he just kept crying that he's not crazy. I know therapy is an important part of what we need to do to help him but I've never dealt with a self harmer. Is it better to hide all the things they could hurt themselves with? Take them to a hospital?

@Mommaof4 sorry if I explained the alcohol thing weird. The best friend snuck in the alcohol. Not in a roofie way. In a make sure the parents don't find out way and they all drank. My son got drunk and fell asleep. I wasn't home and his girlfriend and his best friend went in my bed and had sex. Ew. But I guess he had also been hurting himself before. Him saying he should have listened when everyone told him she was a "slut" is because she's known to cheat." I'm still grateful she told me. Just wish I had a few more details so I didn't trigger him by having her there.