First of all get rid of WoW and any other RPGs... He'll be pissed off for a week and then forget about it. Buy him a bike instead or something.
Secondly he's clearly just totally unconnected with school work and doing his homework for him is not going to help at all, especially since he's just plying WoW instead. You say he's intelligent so he shouldn't really find GCSEs too hard if you can get him interested, and he's got a fair few months yet to get it together... Sit down with him and talk about the subjects he's doing, think about ways of making them interesting. By that I don't mean 'science can be fun!', I mean watch some David Attenborough documentaries, get hold of those programmes Marcus Du Sautoy did about the origins of maths etc (actually there's a programme on iPlayer where he looks at consciousness which looks quite good), basically show him that these subjects are far more interesting than the snippets you learn at school. Maths especially is a subject in high demand with some genuinely fascinating career paths at the end of it. Is he interested in physics too or is he blighted by 'the physics teacher problem'?
He's 15... You need to remember that; did you have any idea what you'd end up being at 15? I was doing something similar at that age, just not engaged... However I had a good support network both from teachers (private school, had very good ones) and family. My mum was single and working as a self-employed architect (ie epically long hours) but managed to find time to help me and my sister out. Also an uncle who was very close to the family (both of their spouses died) and a good designer. It helps so much having passionate and engaging people around because it gives you a kind of vague aim in life.
Basically what I'm saying is you need to help him, and by doing this kind of stuff for him you're essentially creating a self-fulfilling prophecy; for me it would have been horrible to have the people closest to me telling me that I didn't really have much hope except in manual stuff (which I love and actually work in, but hey). You're viewing school as this kind of thing that he has to get through before he starts work, and to me that's a very negative way of communicating to someone who, as you say, is a) intelligent and b) enjoys maths. He may well be sitting there thinking 'well I'd love to be working on the large hadron collider but no-one would take that seriously'.
Sorry, I really have no idea what your parenting is like of course, so a lot of that may be unfair. I dropped out of uni (for health reasons largely) and retrained as a cabinet maker which I love (although I've got a job as a robotics engineer at the moment

), and I do recommend that kind of thing, but I'm always glad I managed to get through school and do pretty well (after a few retakes). Partly because it means I can go back to uni, but mainly because I learnt about random stuff and it lead to studying architecture (which, although I dropped out, was fascinating).
Stick with school for now, and try to help him, there's no rush to apply for work after all and it'd just be a lot of added pressure on him. The trades are good with young people as that's where most of them come from, also helps that you get 100% funding to train a 16-18 year old. But there is no need for him to do it now, he can always drop out after AS levels if he carries on with school.
You really need to look at what he wants, not in terms of what career he (or more accurately the adults around him) thinks he should move into, but in terms of what interests him. Yeah, he could go into work and some of the professions are great, but it's not hard to retrain into that kind of thing whereas it is bloody hard to motivate yourself to get back into education. Tbh it sounds like you could make this into a positive thing for yourself too, connect with him on learning about new stuff, give yourself a bit of direction by making it your task to help out and engage more... go for walks with him, have talks as an equal rather than concerned parent.