Desperate for Advice Please

I don't know where to start.

My 15 year old son, diagnosed with Asperger's at the age of 7 is a tearing our family apart.

He started seeing a girl almost a year ago, she doesn't live locally so a train journey is required (half an hour or so with a changeover) and she seems to be controlling his life.

His behaviour these last few months has become unbearable, he steals money from us to go and see her even though we give him money (which he has to earn by doing chores) and he lies constantly. She controls his every spare minute, messaging, facetiming and snapchat. She gets angry with him if she doesn't have his undivided attention and she threatens to dump him on an almost weekly basis. She swears at him, shouts at him and tells him he's a bad person and he believes it. He's been talking about killing himself and will not see what is happening. He is completely consumed by his relationship with her. We've tried talking to my son, tried sanctions, ground rules and compromise but he is totally obsessed with her and being with her even though he knows she treats him badly. 

Sadly, I think her behaviour is learnt behaviour, she witnesses her mum arguing with her dad, he gets thrown out, stays in a hotel and is only allowed back the next day providing he's sufficiently sorry enough. My husband has met her dad and said he seems like a nice guy.

My son has panic attacks and meltdowns, he hates us and we just can't reach him.

He lies constantly about everything. Yesterday, he was supposed to be at an after school athletics event, I went to pick him up from school after and knew something wasn't right. He'd come out with all this information about the event, where he was placed, what events he did and everything. I knew he was lying. Eventually, he said he went to the park (a train ride away) with his friends from school and that he'd lied because we wouldn't have let him go (he was grounded from the weekend when he refused to come home from his girlfriends) and that he wanted to spend time with his friends and be included in things. He even got one of his friends to lie to me on the phone to say they were together. I found out this morning that he wasn't with his friends at all but with his girlfriend. 

I've been to my GP and asked for a referral to CAMHS but am still waiting on an appointment for him. He gets so angry and abusive and he goes on and on for hours on end to try and get his own way. I'm worried about his mental health and think he needs help but he's refusing to go anywhere. 

I've told him that I will call her dad if things continue as the relationship is so destructive but he says if I do the relationship will definitely be over and he'll kill himself. I just don't know what to do. We can't just let him do whatever the hell he pleases but we also just can't reach him with conversation. He refuses to accept sanctions and will not accept that there are consequences to his actions.

I'm sorry for the very long post, and I haven't explained even half of what is going on but I'm at the end of my tether.

Thanks for reading.

Nicky