boy, oh boy, do i feel ya. what a rock and a hard place.
marsha linehan once told a client that while she would provide the upmost support, she couldnt prevent her (the client) from committing suicide. its advice that is more cold and clinical than i would tend to prefer to offer, but it is part of the reality of loving a difficult and troubled person. you may see marked and dramatic improvement, but they will, likely, always struggle.
having said that, getting
faced and calling someone up, with depressive or even suicidal thoughts, isnt uncommon for someone who struggles; its not even unheard of for someone who doesnt have bpd.
and i dont say that to downplay what he said or what you experienced. it sounds like you handled it brilliantly. im saying it can be difficult to distinguish from the struggles our loved ones have, have had, and will have, and a person who struggles and is on a really bad drunk.
and really none of that speaks to the fact that a suicidal threat is a cry for help, and a bad drunk doesnt diminish that; it is, in itself, a cry for help.
the easiest thing is to talk to him about your concerns. how to do that is not necessarily as clear and very much depends on your relationship. has anything been said about it between the two of you since?
it might be a good idea for the two of you to come up with a plan when (if) these things happen. it might be a good idea, to the extent you are able, to discuss how you feel when these things happen (tread lightly, obviously. you want to be able to communicate, you dont want him to walk away with the impression that he shouldnt share when hes struggling).