Is he getting any help from the CMHT? The quality of services available for people with BPD varies enormously, from nothing at all through to very well set up appropriate treatment, but even if your area is lacking in specialist treatment the CMHT should be able to offer some support.
I'm not sure what to suggest in terms of boundary setting etc - I have a BPD diagnosis but from what you are saying it sounds like your son presents in a very different way to me (not uncommon, as BPD is quite a broad diagnosis). Mine primarily centres around low mood, some difficult relationships caused partially by fear of rejection, lots of feelings of being worthless and inadequate and various other things - I don't drink/take drugs/shoplift/get into one night stands/frequently self harm, which are a lot of the things people commonly associate with BPD. I have always tried very hard to not let my problems impact too much on those around me, although of course at times they do, particularly if I am very unwell.
Try to be supportive - I am sure it must be really difficult when you feel like he is acting like a spoilt brat and being deliberately awkward, but he is ill, and probably really struggling with his illness. It is a very isolating, difficult illness to live with, and he probably hates the way that he behaves at times (I know I do anyway). Also, you said you don't know what he has to be sad about, but frequently people who have BPD are also depressed, and as I'm sure you know that doesn't neccessarily have to be 'about' anything. You can't fix it for him, and it will probably be an issue for a long time if it is BPD, but over time hopefully he will develop insight into his problems which should make things a bit easier. Remind your husband and daughter that he is ill, not just being a twunt on purpose, but also tell him that he needs to take some responsibility for his behaviour, and that being ill doesn't give him permission to behave like a twunt!
There are various books around - some are awful and some are very good. It may help him to read some too; you can feel incredibly alone with BPD, and just seeing that there are other people experiencing the same kind of problems can be really comforting. Also, you have probably already read it, but there is this info on the Mind website www.mind.org.uk/help/diagnoses_and_conditions/borderline_personality_disorder