Hi Kanga,
Along with
Wendydarling I to would like to welcome you to the group! I read your post this morning, but unfortunately didn't have time for a thoughtful answer but I have been thinking about you today.
Your son and your family reminds me of mine, 13 years ago. The way that you describe your son is so similar to my son, and my daughter like yours has no history or indication of mental health problems. My kids also have a father with BPD traits.
I agree with Wendydarling that there is good news here. It sounds like you have done a great job so far and the fact that he is off to college is a very positive thing. Also, the fact that you describe your husband as "Owl" and use the word "we" when you describe handling things makes me think that you have good support from a wise husband. My husband is awesome, but I couldn't refer to him as "Owl"

Your concerns about suicide ideation and loneliness in college are very valid concerns. As Wendydarling said, many people (especially us with BPD children) have worries when our adult children leave the nest. We are suddenly no longer in control.
This is going to be long but this is what I would like to say to you (mom to mom):
We need to take the worry and anxiousness, and turn it into something else. I don't have a word for it, the best I can think of is "concern with confidence".
It just reminds me of this: my husband and I like to do certain sports that others consider "extreme". When we meet people wishing to learn we always teach them that it's about learning to respect the danger, not to be fearful of the danger. Being fearful of the danger does not let you progress, and could cause problems. We need to be mindful and respectful of the danger and the only way to not be fearful is to learn all of the skills and use all of the tools available.
So, back to your son. Your concerns are valid, they are real. Now what?
Now you learn everything that you can, and find all of the tools that you can. Go on multiple suicide prevention websites and learn what to say and what not to say when someone is talking about suicide. Practice with your husband. Learn everything that is here on this website and in books about communication.
Stay in touch without looming over him and have confidence in him.
Honestly Kanga, from what you wrote it sounds like you have already been doing that and now you are going through a transition. Transitions are hard, and new skills and tools have to be developed.
It is often said in the BPD world that "everyone is doing the best they can, and everyone needs to do better". I commend you for how well you have done so far. You will get through this!
Oops - I wrote this last night and forgot to hit send...!