Your situation sounds so awful and heartbreaking and I'm so sorry that you don't have any family support, it must feel very lonely and frustrating having to deal with this on your own.
It sounds like you've read the situation well e.g. if you bad mouth the GF that will push him further away. But it must be very hard to watch your son change and do things that are not in his own best interests.
My sister ended up in a similar situation with her son although the GF wasn't self-harming, but she was extremely controlling. Sadly, my sister dug her heals in and made the mistake of telling her son that the GF wasn't right for him etc., etc.,. Eventually her son married the GF — and to cut a long story short, my sister has no relationship with her son now, which is not her choice. Her son's wife I think has symptoms of Narcissist Personality Disorder shown my her lack of empathy and conceited manner: underneath it all, I think she can't bear anyone challenging her own delusional view of herself — and he's bought into it. Although my sister was married, her husband was pretty absent / ineffectual (an alcoholic) and she had brought up her son by herself. Until the GF, they were extremely close, she had a say in everything he did — some family members have described my sister as controlling. I think of it as part control and part mutual dependency. But as soon as her son reached his late teens, started Uni, got a girlfriend — that dependency shifted to the GF. His dependency shows how unable he is to stand on his own two feet — he went straight from his mother to his girlfriend (now wife) with no period of independence in between. I now often think that my nephew chose a far more controlling version of his mother. He's incapable of thought that's independent from his wife. They even share a FB account — a merging of both their names. He also behaved very differently in our company — and if she was present, or at the other end of the phone, his whole manner would change. I sometimes think that my nephew has some kind of Stockholm Syndrome if such a thing exists in emotionally abusive relationships!
For whatever reason, your son seems to be dependent on his GF. Has he ever been confident, independent in his own right? If he behaves 'normally' with you when he's not with her, I'm guessing he does that because that is your 'usual' way of being together and she's not there to apply her judgement. Even though you know that this relationship doesn't appear to be doing your son any good — it doesn't mean that he will see it that way. At that age, there's probably a multitude of emotions and things to prove — and you say she's his first GF. He may be so thrilled to have accomplished getting into a relationship that he's willing to sacrifice all sorts of other things to maintain it. Until, hopefully, he sees the light. It's worth remembering that first relationships are rarely the lasting one's.
In your situation, I would make it my sole aim to maintain a relationship with my child. I'd try very hard not to criticise him wanting to see the GF (wondering if he causes arguments as an excuse to get out to see her because he knows you don't want him to?), I'd try to keep communication open even if it means biting your tongue. You can't look out for him if you don't see him. Remind him that you love him, that you're there for him and that you care for his well-being. In the midst of all this tension I know it will be difficult but I would try to find opportunities to remind him of his good qualities — it's easy to let a difficult situation dominate all your interactions and he may start forgetting what he's got going for him independent of the GF. Don't get sucked into situations where he's trying to cause arguments — he must know how to push your buttons. Reasoning that you haven't done the things he says won't necessarily make a jot of difference if his aim is to do all he can to see the GF.
I'm interested to know, do you try to stop him seeing her?
I'm not sure what support exists for parents of teenagers but I wonder if there's someone you can talk to, with some professional insight into how to cope with this?
Anyway, my heart goes out to you.