Good morning @Leigh and thank you @CherryBomb for the mention in your post.
Gosh, where to start with BPD. BPD is one of those mental illnesses that keeps you guessing all the time. One day the person that has BPD will act one way and the next day it's all different again. They are never on the same wavelength as you.
My sibling also thinks that doctors and psychs are useless because they don't know anything and can't help in any way. It wasn't until my sibling ended up with several psych ward emergency visits that the doctors started to sit up and take notice that something wasn't quite right. By the way, most psychiatrists and doctors hate BPD because it can be quite difficult to treat. No kidding!!! Your son kind of has a point because some of them really don't have much of a clue when it comes to BPD.
The book "Walking on Eggshells" that @3forme mentioned will give you some idea about what's going on in your sons head. For us normal people it's all quite bizarre. When I first tried to read the book it totally defeated me. The information in it was just too much to take in. On my second attempt I got a bit further. A year on I have nearly gotten all the way through the book but I also have a much clearer head now that my sibling is on medication. When you're in the thick of things, with the abuse and horrible behaviour you can't think straight anyway and the book confused me more than helped me. I don't want to put you off in any way and if you do read it I hope you have better luck than I did.
I absolutely recommend seeing a counsellor for yourself - someone who has some knowledge about BPD. I mean, if some doctors can't even deal with people who have BPD how the hell are we meant to.
Boundaries, boundaries, boundaries. I can't say enough about getting this right. I didn't have any when my sibling went completely nuts and boy was it to my own detriment. Learn as much as you can about setting them and sticking to them like glue. Boundaries will help you keep some of the sanity you have left.
Learn how to say no. When the abuse starts you say "no, I'm not listening to this until you can speak to me calmly" or "no, I do not have money to give you so stop asking me". A no does not need to be explained, no matter how the other person hounds you for an explanation - and believe me they will. When you say "no", then that is the end of the conversation. No ifs, no buts. Your no should become gospel. It's an amazing boundary to have. Ignore the temper tantrum that follows.
Dealing with someone who has BPD is challenging at the best of times. It's almost like dealing with someone who's 5 years old. You need to make yourself very clear about what is allowed and what isn't. Don't ever say something that can be thrown back in your face. Don't ever promise anything you can't deliver. When you talk to your son be very, very clear in what you want to say. Then ask him to repeat that back to you so you know he understood. If you don't understand him (which will happen often) make sure you ask for clarification, even if you end up sounding like a broken record going over things a few times.
I also can't emphasize enough the importance in your son seeing someone who specialises in BPD. There aren't many out there (because BPD is quite often put in the too hard basket) but be persistant and do what you can. Many of the carer services will know of people who do specialise.
The substance abuse isn't going to help him either but I'm sure you know that. Is he getting any help in that department?
In my experience BPD can make people do evil things. They will do things on purpose to hurt you. My sibling has plotted and schemed behind my back to carry out the worst things against me as well as my family. If this ever happens to you make sure there are consequences or it will happen again and again and again.
Can I ask you how much your husband has seen or heard in regards to what's going on with you and your son? If he knows everything then really, I would expect him to put his foot down and stand behind you 100%. If he is not completely aware then you need to start telling him every little thing that's happening on a daily basis if it's that frequent. Do not isolate yourself. You both need to represent a united front.
Also, what medication has the GP put your son on to treat the BPD?