Finally, at around 23, our BPD daughter had a complete meltdown due to her own destructive patterns. . . . We admitted her to an in-patient program just like we had dozen of times in the past. But this time, she was so sick and tired of feeling the way she felt, that she actually wanted to change. So she actually did the work, actually accepted that so much of the problem was her.
I just want to mention that the situation with my pwBPD (my adult stepdaughter) was similar. While her BPD behaviors emerged in her teens, they got progressively worse, expecially after she left for college. I think that was because in the "real world," for example in her college dorm, her nasty, entitled, angry, explosive behavior would not be tolerated the same way it was in her family. Like Pook said, the situation seemed to get worse and probably reached its nadir at around age 23. Ultimately she was admitted into an in-patient program, just like she had done a handful of times in the past, mirroring Pook's experience.
But what was different this time? I think that she "hit bottom." And importantly, SHE was the one to admit herself to the program, not her parents. You see, on previous occasions, her parents were the ones to take her to the hospital after experiencing a major meltdown, at least once requiring police intervention. But of course, she flipped the script, blaming her parents hor "forcing" her to go to the hospital and accusing them of "assault," for example. She maintained that her parents were the sick ones (note the projection). And though she would be complete the involuntary holds, she didn't consistently do the recommended follow-up therapy. Note that during one prior hospital stay, I think she pretended to "go along with" a treatment program only to win a concession out of her dad (for him to pay for her own apartment at release). However, when she finally took herself to the hospital, I think she was "ready" to work on herself to get better.
By that time, she had gotten an ultimatum from her doctors, too: either she participate in the recommended therapy program, or she would have exhausted all other options, save for long-term involuntary commitment if she were ever to land in the hospital again. In essence, she could choose either to accept treatment or lose her freedom. Her dad gave her a simiar ultimatum: she needed to follow doctors' orders, or she could go her own way (and lose all financial support from him). Fortunately, since she had hit bottom, I think her choice was an easy one. I'm glad to say that she managed to turn her life around in a relatively short time. Though she has experienced some setbacks and struggles, her life looks much, much better now. I only wish she had hit bottom sooner, and that her parents hadn't enabled the unhealthy status quo for so long.
I'll add that my BPD stepdaughter seemed to warm up to the notion of getting professional help. That validated her victim narrative--that she felt so traumatized by life that she needed professional support to cope. Once I said something like, "It's really mature of you to take advantage of the professional support available. Doctors know what to do, they help people help cope with trauma all the time, that's their job." I think my husband liked that approach, too, because he could let go of the responsibility of trying to "fix" his daughter, as nothing he had done in the past had seemed to do any good. Once I told him, "Your focus right now is to make sure your daughter follows doctors' orders." Therapy became her number one priority for a time. Everything else--college, travel, living independently--could wait.
I happen to side with Pook on the need for boundaries, so that you don't enable an unhealthy status quo and ruin you life right along with your son's. I know, it would be incredibly hard to kick your son out. I'd advise not to threaten to kick him out if you're not prepared to follow through. My husband asked my BPD stepdaughter to leave our home last fall, because she wasn't abiding by the house rules. To live in our home for free, she was expected to be nice/respectful, pick up after herself (the bare minimum, not even clean her own bathroom!), and get on a path towards full-time employment, through some combination of looking for work/interviewing, part-time work, training, internships and/or volunteering. (She had quit marijuana at the time but another house rule would be no drug use on the premises.) Of course, she's furious about being kicked out, but at the same time, she barely worked/looked for work when she was with us, and instead she was gradually resuming her old habit of sleeping the days away. Once she was kicked out, she had no other choice to get a job and find a place to live. At first, she landed on a spare couch, then she house-sat, and finally she rented an apartment with roommates. Though I'm pretty sure she hates us right now, I think that kicking her out was one of the better things to happen to her, because she was forced to live in the "real world" as well as truly start her adult life. I'm proud of her for that.