Totally agree with BarbarianMum’s good advice.
“Maybe you need to start again with him. Ask him what he wants to try instead of school. Find something (as well as xbox) that will boost his self esteem and at least help him develop independent living skills and baseline work skils (getting himself places on time, dealing with people, motivating himself). I doubt it'll be easy because he will likely be very negative at anything you suggest but it can't be worse than the current situation.”
IMHO I would also start with what you know. "I understand why he doesn't want to go to school, he doesn't want anyone to see him to take the piss out of him."
So begin by talking to him about why he doesn't want to go to school and instead of trying to chivvy him into going (which I expect others have done in the past), really listen to how he feels and why.
Next ask him what he thinks he could manage, bearing in mind being at home and watching TV/games is not an option.
Just an interesting point, my friend had a child on the Autistic spectrum and she says that certain things help more. 'Weight' can help, as in carrying a back pack around, being under a heavy blanket. This is to do with sensory issues. It may or may not be relevant.
www.medicaldaily.com/kids-autism-weighted-backpack-sensory-overload-379901
I think that homeschooling can lead to better opportunities. I think you need to find a way to barter with him to get him to do what you want! Some will disapprove but for some kids this is the only way. Take what he likes treats/computer games/TV and use these to get him to do some things that are good for him.
Do not ration love, understanding, chats, hugs or family time, but do ration TV and internet. Explain you are in this together, him and you, he may be able to understand that. You may even need to employ some suitable language like ‘This is a quest and I am in it with you, how are we going to plan the rest of the school year etc.’
Keep notes of what works and what does not work. Use the internet as a resource to get new ideas.
I am sorry if this all sounds very obvious.
I just really do not want you to give up on him because I know you will regret it.
Lastly, here is the thing, you need to barter with you too. Reward yourself with a swim, or spa day (if you can afford it), some chocs, a long bubble bath whatever, every time you go the extra mile for him. This is really how much of the world works, fun things, difficult things, really crap times, rest, relax, back to work etc. It sounds like yours has been a lot of really crap and difficult things. You mention other kids and maybe this feels like a lot of work.
Can you other 14 year old help you a bit, with stuff at home so you have a bit more time for you?
Totally agree with wombling “I don't understand why the school are expecting a child who has SEN and is so academically delayed, to be able to take GCSEs. Of course he will fail them if he is 4-5 years behind.” The school are actually the ones failing him!
Point this out at this meeting. It is like expecting a child in a wheelchair to run a marathon.
I agree with “I'm sure there are groups and online help for home educators of children with SEN. Rebuild his self esteem. Get his reading, writing and basic maths consistent. Set small goals and reward them with X-box time.” However, IMHO, first make sure there is no chance of a special schools place.
Good luck. 