This sounds a bit nosy of me, but what do you think was the trigger for his suddenly disengaging with education? since his igsce's, which presumably he did in school. Was it something that happened in the last term of school, or the holidays setting a pattern of isolation, or just the stress of getting through the exams and feeling he can never get the same good results a second time.
I have a son in home education who is 13, with ASD, and I want to try and make sure that the current plan to send him back to school is the right one. At the moment he isn't anxious, but a bit bored by lack of structure and motivation in a home setting. I want to do the right thing - give him academic opportunities but not exacerbate anxieties. He goes out a lot and sees people atm, and we do lots of activities, but he is very behind academically in lots of ways, despite being average intellectual ability.
I so sympathise with your son, I remember in a small way that feeling of not wanting to see anyone aged 17, curling up in a ball, feeling I couldn't face school despite being successful academically, and pressures of course work etc. I think it is a pattern that can recur throughout one's life, but in my case a structured social activity always saved the day..I think I did a drama club (I was good at singing)and that brought me out of my shell. Sometimes a very small change, a chat with my parents in the garden at breakfast, a visit from a relative who was detached and not interfering and told good jokes, or a reason to go outside however briefly can tip one back into feeling more optimistic and then another tiny step forward is taken. But I can remember feeling there was no way out, and I wanted to die - this is for reasons of social isolation etc. All that seems alien to me now, I think those feelings are particularly strong when you are teenager, with any social communication issues and low self esteem for various reasons. It is an impossible vicious circle, you begin to imagine that everyone can manage life but you cannot.
For me the most important thing was to be forced to talk to family members occasionally, at mealtimes and not encouraged to be alone or retreat. Is there any way you could tempt him down in a very non critical manner to share food? Or do some small task for you or errand to rebuild esteem? Or watch humorous telly with him and hear his opinions.