He seems to have too much control over his life, in the typical way teenagers do, i.e. without the responsibility or awareness of consequences.
Have you lost out financially from him losing his boarding place?
It sounds like the boarding was working well for the family as a whole. Is there any chance that if you secure some promises from his re attending lessons they would have him back? Maybe the school have lost faith in your ability to sanction his behaviour and a show of strength would help, particularly since it doesn't sound like he has been disruptive.
YOu say you can't withhold privileges, can't ground him, etc. because that won't work. So, he has bunked off school and is still getting all his stuff? I would cut off all his privileges and set out what will happen since he doesn't respect you and what you give him. Take him for an appt to look around the local school.
I would get a bit crosser.
I have a had a school-refusing, difficult and rude teenager who is now working hard and doing well, mainly because we made it very uncomfortable for him do things any other way, but he was younger and I do realise it gets harder when they are nearly grown up, physically at least .
Do you have a parent support unit anywhere near you? I think very understandably, it is all starting to feel like 'he won't' and 'I can't' and talking it through, with or without him, may give you some clarity.
Good luck.