We don't want to lose our son

So this is going to be long, but please bare with me. I just can't shake the feeling this is an unhealthy situation for my son to be in and will not end well.

I'm a mother of a 16 year old boy. Normally I would say he is a sweet natured boy with a heart of gold. He's quiet, shy and a people pleaser. But lately, there is not much of that boy showing. He met a girl online 4 or 5 months ago. He had not long turned 16 and she was 18. My husband and I found out that this girl was also flirting and speaking in a sexual manner with our 14 year old son. So we enforced a complete ban on both of our boys speaking with her and she was also told to stay away. We felt this was enough, because she lives in a different state to us.

She was blocked on all devices, we even got rid of the PlayStation and now only have an Xbox, in our bedroom. Our 16 year old was dirty on us for about a week, as we continued to explain our reasonings to him. We explained why it was wrong and her speaking with his younger brother was not only wrong, but illegal. He seemed to understand and let it go. Like most 16 year olds, he began spending alot of time in his room and not spending time with any of his family. We let that slide, because he's a teenager and we thought he was still pissed at us.. he thought he was in love with this girl. He has referred to her as "his everything" & "his life". Fast forward to now, we thought everything was going better, he had met a new girl and seemed to be happy.

In the past few weeks we have discovered that this girl is also not a good influence. Our son has started to masturbate, which again is normal for a boy his age. But where it becomes unhealthy is she is on the phone or facetiming with him and telling him to choke himself and not let go until she says. They have also been talking about cutting for sexual pleasure, which she has found a "safe class" that give instructions. I started to suspect this was the same girl we asked him to cut contact with. He showed me a photo of her to try and prove it was a different girl, she looked similar, but with shorter hair. I put it down to him "having a type". Last night that was confirmed when I came across Reddit posts my son has made and I searched his phone. She has given him a sob story of being sexually abused by every guy she has been with, her parents are abusive etc and he is her saviour. She tells him she needs him to call her and wake her to take her medication, she needs him to stay on the phone, even though she is asleep and call her whenever she asks, because otherwise she will kill herself. He has apparently talked her down from suicide at least once. Secretive is best because his parents don't need to know, he's 16 and legal so it's none of our business.

He's talking of getting a job and when he is 18 leaving "this shithole" and never coming back. Regardless of what we say, he sticks with this girl. Thursday night I found this original girls tiktok and now know for sure it's the same girl he is still talking to. I gave him the opportunity to be honest with me and he still claims this is a different girl.

So far they have been talking and participating in SnM, bondage, cutting, video call self pleasuring on the other person's say, they refer to each other as sub, master, etc. She has told him she wants more than one guy, his reaction to me telling him exactly what that means had him almost vomiting, yet he agrees with anything and everything that she says.

On Friday he started sending me abusive messages whilst I was out and demanding I tell him what I knew, I tried to get him to wait until we got home, so we could have a proper discussion about it all. He refused and started throwing insults.

When I arrived home he was straight up in my face, verbally abusing me and demanding to leave. My husband and I told him he could go, but wasn't to take anything that we purchased with him, including his phone. He became angry when I tried to grab it and he grabbed me and threw me on his bed. I then kept my distance from him. He ended up leaving and leaving his phone on the kitchen bench. He was missing for about 4 hours, when he found a stranger and got her to call my cousin. He is now staying with my cousin. Today is Tuesday and even though we have shown him messages from this girl saying he can't go to her place and she doesn't want a relationship with him, he is still adamant that he wants to leave us to be with her. She has said that she is almost 19 and he is only 16 and that's gross. We allowed him to message her from my phone, where she then told him that she loved him and missed him, sucking him right back in. If we cut the contact, he will likely run again. But allowing him to have contact with her, when she has already been inappropriate with our younger son, is not an option.
We have since found out that this girl has an onlyfans site and is highly sexually motivated. Our son had also downloaded an app to remotely control her vibrator, along with having multiple emails and encrypting his phone so that with the password he shared with us, certain things would be hiden.

We feel she has groomed our son for her own gratification. We feel this is an extremely unhealthy relationship. He's our eldest son and we are so lost as to what we should do.