I just want to say that I really feel for you. I'm not going to say just go ahead and pull your son out of school, but I will say that it doesn't sound to me like you have much to lose other than a very miserable and distressed child.
I cannot tell you what to do, so this is my personal opinion and partly comes from my own experiences. I am a single mother also with 2 HE'd boys 13 and 10 and face the problem of working too. Am (rather unsuccessfully so far) trying self employment too. Finances are dire.
If I were in the same position as you, and I suspected that my son was about to fail his GCSE's massively after a very wretched and unhappy few years in school leading up that point, (i.e. not just messing about or laziness) I would take him out. I would not risk the potentially tremendous damage that failing his GCSE's would have on his self esteem. There is nothing quite like the blow of seeing everyone in your age group crowing about their passes, when you have tried really hard but 'failed.' I would spare him that potential experience with the inevitable dive in his self confidence which could hold him back in life.
I would rather he concentrated on doing a couple of GCSE's in core Maths and English at home as a private candidate over the next year or so, and let him study everything else in his own time. Better to taste success in two subjects than fail everything. (there is an EXAMS list for home educators/children studying for exams at home. Someone here might have the link or if you're interested I can find it for you later) You may find his interests lead him to other things. Some children have got on to courses that interest them without the full requisite GCSE's, my point is that if he doesn't do well at school anyway you really don't have much to lose.
With regards to him 'not liking being taught,' that is understandable, but for my peace of mind I would tell my son that he can only come out if he agreed to me helping him with his English. I find now that I don't really teach so much as work alongside my eldest, sometimes that means pretending I don't know the answer to something I really want him to learn and we end up solving it 'together' LOL.
If you decide to go ahead, working and being a S.P. will probably be the greatest difficulty. At 14 he can be left alone for short periods but only you can say whether you'll be able to arrange some sort of care for when you're away. With regards to his studying you can be as flexible with that as you want -I sometimes take out the chemistry book on the train to somewhere or we do nothing on a Friday and use our Saturday morning or Sunday evening instead.
Finances will be stretched, but how much you spend is up to you. Ask for stuff you really need at Xmas, the library is good, and I have an account for exams which I put in spare change.
Also some find that the social aspects for teens being Home ed are a challenge, it has been that way for us. That is something else to consider.
HTH a little