Hello and a warm

You're among others who really understand the challenge of this question -- how much, or if, to support an adult child with BPD. This is not easy!
Just a few questions to help me get a feel for your situation:
How did your S23 get diagnosed (psychiatrist appointment? mental health crisis team? something else), and is he accepting of the diagnosis?
Is he currently living with you, or elsewhere?
Does he accept that he has a drug/alcohol problem, or is he in denial? Has he ever gone to rehab, attended AA...?
Did you financially support him while he was employed, or did you start recently (when he quit/lost last job)?
What does he spend money on?
Did he say why he is stopping therapy and meds (regardless of whether you agree with his reasons or not)?
...
It's hopeful to hear that you want to want to maintain a good relationship between you and S23

As you're probably well aware, maintaining effective and positive relationships with a pwBPD isn't always easy or intuitive... but it is possible. A good place to start on our site is our section of articles on
When a teen or adult child has Borderline Personality Disorder, especially (even though S23 doesn't seem to want therapy right now) the article on
Supporting a Child in Therapy for Borderline Personality Disorder, which has some great "mindset adjustment" ideas, even ones that seem counterintutive.
When you have a moment, take a look at those, and I'd love to hear if anything really stood out to you.