Hi Frustrated,
Welcome and hello
Walking on eggshells is a pretty common phrase around here!
How does your S20's BPD traits present? Does he externalize? Internalize?
Is he diagnosed for BPD or anything else?
There are counter-intuitive skills to help prevent things from getting worse, and some that are about learning to coach (instead of enable or rescue or save, for example). It takes a while to get the hang of things and small changes in our behavior can make a difference in theirs.
Ultimatums don't tend to work. We talk here more about setting goals and having boundaries to protect ourselves. Sometimes, first things first, we have to learn what BPD is to help us understand what we're dealing with.
Glad you found the site and decided to post.
We are here to walk with you.
LnL