Trish, this is one of my biggest fears I think. I have watched my parents struggle with my younger brother (who is now 21) for years. He has been in and out of courts etc, He is currently serving a suspended sentence. the amount of crap this kid/bloke has done over the years is just amazing.
Worst of all he is usually the older one of the group so while he gets charged the others are let off with warning etc.
His worst was being involved with a group of "mates" who decided to go mug some people, each of the 3 victims say that my brother wasn't involved, just stood off to the side. But he was there, he was the only one over 18 ( I think he was 18 at the time), he was with them when they got picked up etc so he is n ow "doing the time"
My brother does have mental health issues, depression, anxiety etc. He also has learning difficulties. he ahs no concepts of cause & effect. So he is the perfect "sheep" He does these things just be a part of the group.
Unfortantly I have no magic answer to help you & say this is what works because so far, nothing has worked for my parents dealign with my brother. they bail him out time & time again. Even after a 2 week stay in silverwater jail, he still hasn't really learnt.
His crimes have settled down to mostly driving offences now, he no longer has his licence. But I think this is just because he has no way of getting out with his mate etc. Has no car, no job, lives at home.

The only thing I can say is that maybe sending him off to "the naughty boys home" (lol I have to laugh at that as mum use to always threated to call them to come get my brother when he was little, if only she had LOL) may be the wake up call he needs. He ahs pushed you all to far & what else can you do? It sounds as though he thinks its all empty threats. Maybe a good scare will bring him back to earth KWIM?