17 year old isolating himself completely

My seventeen year old son has isolated himself completely. He has no friends whatsoever and he constantly clashes with his immediate family. He was the first boy in a large family, always timid and prone to bullying, but nothing out of the ordinary. In primary school bullying got worse, but as he grew older he learned how to fend for himself, and later on be quite assertive. He always had very few friends at any one time, two or three, and they always drifted away. On the other hand he's always been the teacher's pet, they generally praising him and lavished attention on him. Not because of him playing the victim, but because he always showed respect to them, worked hard and had a genuine thirst for learning. Up till last summer, he had two friends with whom he'd go out occasionally but this stopped completely a few months ago..... What I found heartbreaking though, and what drove me to ask for advice, was a school Christmas dinner he attended a few days ago. On his mobile he had just a couple of photos of the view from the restaurant, no group photos, no selfies - nothing. I still ask him to come to family outings and engage him in my hobbies but he's overly confrontative with everyone, annoying the hell out of his siblings. When we ask him to stop, in a calm and nonchalant voice he'll answer 'what are you going to do to me if I don't?' It's very unlike him - or I don't know my son at all - to do this. He goes to extremes to have his way even in the most trivial matters, and retorts 'this is why I don't like hanging around with you' when his younger siblings (who often handle him in an amazingly mature way) or I try to reason things out with him. He ends up spending days completely alone and while he behaves normally at home any little thing can set it him off...... anyone been through this or knows what's going on?? via /r/Parenting http://bit.ly/2Ropj1H