Have the two of you ever had a successful conversation about past events (behaviors & emotions, whether his or yours or both)?
If so, what was necessary to have a successful conversation? Are there known variables (i.e., needs to be longer than 2 days after the event, needs to be out of the house in a public place, etc)?
Many pwBPD, after a time of dysregulation or heightened emotions, can take longer than other people to return to an emotional baseline. Sometimes I hear things like "if there's an argument, both parties should take a break for 30 minutes to regulate, then return", and I'm pretty sure that is much too soon for a pwBPD.
Do you have a sense of how long it takes him to get back to an emotional baseline? Could be hours, days, weeks...?
When he's regulated, does he ever talk about shame? Do you have a sense for how much shame impacts him (not much, a bit, very much, all the time, other)?
This of course, leads me to consider several things . . . he has few friends, and he's comfortable talking to me a length . . . so this is to assuage loneliness while he looks for greener pastures. He genuinely loves me as he's stated and this is just par for the course with untreated BPD meaning he's prone to wild swings and unhelpful patterns of thought.
At this point I'd be leaning towards the last option -- this is just how someone with untreated BPD shows up in a relationship, where the emotions of the moment are what are "real for all time"... until there is a new emotion of the moment that is also "real for all time".
I think that when he says "he has never felt a connection", he isn't trying to be dishonest, it's just that in that quick moment, that's how he feels. And, that feeling can change.
Does he do behaviors that are also concerning, or go against your values (known cheating/infidelity, for example)? Or at this point, is it more that his behaviors are essentially committed to you, but his words vary wildly and unpredictably?