I have a relative like this.
Only he's 34 now and been at it for 15 years. It's worsened massively in the last 18 months. He's been violent towards his parents. I think he's been diagnosed with schizophrenia. My completely uneducated observation is that he has pathological demand avoidance. It's amazing what discomforts he would live with in order to avoid doing anything.
What I don't think you or any PPs have suggested is that he goes and gets a job. Any job. Putting screws in boxes. Stacking shelves. Pushing trolleys, whatever. Something that tired him physically and mentally, where he has to comply with someone else's rules, and builds different relationships.
I think that a massive part in this situation was that the parents always thought their DS was above those kinds of jobs. I wonder if you and your DS think this too... a third attempt at A levels is unusual, you mention skiing, sailing and rugby, all quite MC where we are.
My ultimatum to him would therefore be this:
- By Easter, he's either fully engaged with school OR he leaves
- If he leaves school, then within 2 weeks, he must be doing a job where he works 30+ hours per week. Any proper job will do.
If neither of these happen you will then give him 2 weeks' notice to leave home.
Whilst he's at home there will be no WiFi at all, he must keep regular hours (I.e. hours of 11pm-5am are for sleep only), he must contribute x hours of chores per week, any communal areas that he uses must be kept in reasonable condition. He must comply with his meds/therapy etc. Say you'll give him one serious warning if this doesn't happen. If it continues, he'll then go straight to 2 weeks notice.
That's it. What you want is for him to be polite, self-sufficient, and independent. Forget about his education now. If he sorts himself out he can come back to his later... apprenticeship or whatever.
Take the pressure off his education for now. But make it clear you won't support him to bump along. Because before you know it he'll be 30 and you'll both have had a very miserable 10 years.