Hi eri711;
That's got to hurt, to watch your S5 want Dad but Dad isn't around. It makes sense you'd want to find a way to keep that relationship going, at minimum for your son's sake.
That's smart of you to realize that not everything your ex says is a statement of literal truth, or an exact plan set in stone. pwBPD often struggle with
inaccurate expression: what they
feel in the moment is real (just like any of us can feel "overwhelmed" or "abandoned" or "devastated" etc at a moment in time), but how they
express it doesn't always match.
Like I wonder if I say I know he didn’t mean he won’t see our son until he’s 17 (that’s what he said) and know he loves him and wants to be a good dad.
I think part of that idea is on target, and part of that idea might need adjustment to be effective.
Like you mentioned, it's good to realize that he's going to say a lot, but that won't necessarily have any connection or predictive power in terms of what he does (actions).
Trying to get him to see that "you know he doesn't mean it" may be
accidentally invalidating. He may not have awareness of his inaccurate expression. "Pointing out to him" that "he didn't actually mean it" might come across to him like this example might come across to us:
You: "I'm so devastated, my ex has left again -- I'm grieving, and confused"
Me (being accidentally invalidating): "Oh wow, that sucks, but really, I think you're not that devastated -- you didn't want him around anyway"
None of us enjoy being invalidated, and pwBPD are no exception (and may be more sensitive to it than most).
My thought is that when he expresses inaccurately, what may be more effective than "trying to get him to see" would be: validating the valid, not invalidating the valid, and not validating the invalid

No: Invalidating the valid: "You didn't really mean that"
No: Validating the invalid: "It's totally fine to drop in and out of S5's life"
Yes: Validating the valid: "Wow... that sounds overwhelming"
...
One more thought coupled with that.
All parents end up parenting to their comfort/ability level. There's a wide range of parenting ability levels, not only physically, but emotionally and relationally.
Some parents may be more limited physically but have great abilities relationally. Other parents may physically have time/energy to parent, but have emotional limitations. Some parents (not me!) seem to have few limitations. Like water finding its level, parents will gravitate to a level of parenting that they can do.
Your son's dad will have emotional and relational limitations due to his BPD (remind me, has he been diagnosed)?
We cannot make someone parent past their abilities. We may need to learn to
radically accept a parent's limitations (this does not mean compromising on safety). Your son's dad may only be able to be present one afternoon a week, or once a month. This would be a sad loss, and it might also be reality.
What might be more challenging is if your son's dad is impaired when it comes to scheduling and reliability. That is very difficult with young children. I wonder if you and he could find a way where you accept his preferred level of involvement, and as compromise, he agrees to and tries to follow a schedule?
It's possible that S5 might do better with less frequent, but more reliable, Dad time, versus unpredictable "hot or cold" Dad time. I wonder if that's achievable -- what do you think?
I think you're on the right track for wanting to build your skills and tools so that you can be as effective as possible in this situation, for your son's sake
