Can you contact the GP and demand he gets seen by CAHMS today?
Stress that you are very worried and will hold them personally responsible if he comes to any harm.
I've been through it (though maybe not as bad). My high-achieving son, went off the rails after his dad attacked and tried to kill us.
He first started cutting himself (and to be honest CAHMS were shit! They advised art therapy which didn't help one bit).
He spiralled until he took an overdose, then I went out of my way with affirmations of love, sounds crazy, but in his darkest hours, I let him know that no matter what, I would love him.
He did end up cutting his arm extremely badly a few years later, and needed surgery, but discharged himself from the hospital. The police tried to help, but the CMHT were shit!
That day I phoned them and said unless they assessed him and kept him safe I would hold them responsible! They did admit him voluntarily (but with the threat of a section if he left), and he had surgery. It was the wake-up call he needed.
My son has since been diagnosed with Asperger's, and I think he felt a failure, unable to protect himself and me, and was a walking storm of feelings he was unable to express and understand. The world can be a scary place for teens, and they are not able to make sense of their feelings.
Is there anything the two of you enjoy doing together? Try to distract and 'ground' him, until he is able to process how he is feeling?
Sorry I'm not any use, but please demand he is seen and listen to him 