Your last post really resonates with me. We are now a few years beyond the crisis point (16) and things have really turned around (fingers crossed, for I will never take anything for granted again).
What worked for us... Maybe he just grew up, but what I credit as the turning point...just when you feel most like turning him away, pull him close. Take him away with you somewhere you know he will really enjoy. Our son flatly refused to go anywhere with us. But we persuaded him to join us on a few days away, the main highlight Alton Towers (my personal idea of hell, but I went on every single ride) . We stayed at a B&B with great breakfast and we concentrated on food & fun. It was brilliant, for all of us. The little boy in him made a reappearance. And then we began to plot a week away in the summer, really listening to what he wanted and running our options within our budget by all the kids. Learning to drive was another motivating force to clean up his act, get a part-time job and spend time with either one of us in the car... We spun that one out for ages (to make sure he was responsible). It was a brilliant activity & really empowered him.
I know to some this is rewarding bad behaviour (I also know it costs money), but the more we clashed, the more influence other people had over him and the less influence we had. I was terrified, like you, actually ill with it. Show him know how much he is part of your family, how much you love and value him. Small things, like his favourite food & a hot water bottle in his bed - at every turn, no matter his behaviour, show him that you love him and will stand by him to get the stupid English GCSE (that exam is not fit for purpose) and move onto the things he is going to love and excel at. As others have said, concentrate on the forward looking, engaging things to motivate him.
Wishing you all the very, very best. Take care of yourself. Get time to do things you enjoy too - good for your kids to see self-care in action and means you will be able to stay calm (so, so difficult, I know).