Oh Cutted, I'm sorry things are so bad for you and your son.
I empathise. I was also a self-employed lone parent when my son had his worst years in school. That meant I could cope with the occasional exclusions and frequent calls from school without losing a job, but I was always aware of the threat to my business. I had to give up a contract once, and that was a very bad experience for me. You do what you have to do, don't you; but sometimes the stress is hard to bear.
With hindsight, I think my son was quite damaged by his school experience. He under-achieved, but that was the least of it... His self-esteem is very low. He became more challenging. He started using drugs. He learned to expect to be in trouble all the time, and to tolerate being very bored. He is bright - well above average - but believes he is stupid. He is terribly afraid of trying. Today, he should have enrolled in college, and nearly did... but lost his nerve half an hour before we were due to go, and spent the afternoon face-down on his bed instead. Currently, he says he will get a job, but we have no idea what the future holds... As of today, he is a 'NEET', and I am trying not to panic.
If I had the time again, I'm not sure what I would/could have done differently. I did consider home educating him, but ruled it out for personality reasons. I find him so challenging that I need(ed) school as a break from him.
But your circumstances sound different. You don't sound like you find spending time with your son challenging or confrontational. I think, if I had been in your shoes, I probably would have withdrawn my son from school. If I had similar problems with DS2, who is very self-motivated and whom I find much 'easier', I would do it now...
Home educating at 15 is not at all like doing it at 5 - your son has already learned (almost?) everything the national curriculum says he must learn, and you will have a great deal of flexibility. He can be very self-directed. He can do his own chosen projects and activities. If he co-operates, you can plan his learning together, but he can do most of it by himself, leaving you free to work...
With home learning, all sorts of things 'count' that wouldn't in school: cooking lunch is a 'learning experience'; so is listening to a concert on the radio, or a podcast; so is planting and growing things; so is fixing and making anything; so is internet research; so is following any interest or hobby or even obsession... There is nothing to say he must do GCSEs, now or ever.
There is a strong home ed network in the UK. If you haven't already, you could get in touch with someone and have a chat to find out more. I have a couple of friends who have home educated teens, and if you'd like to PM me, I can find out a bitmore on your behalf, and perhaps get some contacts for you.
Sorry if you know all this already 
but maybe some of it will help!