Hi IrisLouise,
He went from adoring me to abusing me (never *actually* hit me) in seven months and seemed increasingly less able to cope with every day life. For instance he didn't work, take care of himself would become absolutely furious over small mishaps.
the problem with BPD is that often people suffering from it can't help themselves and adopt a behavior that is constructive. It is pretty much the root of the problem and you are certainly not alone struggling with it.
He still believes I abandoned him and... .
well, from his perspective he is right. You are not living with him at the moment and he feels left alone. The point here is not to judge you - far from it - the point is here to warn you not making matters worse by arguing his complaints. There is simple no point in doing it - it would be just invalidating. For more see:
https://bpdfamily.com/message_board/index.php?topic=280750.0Right now the most important step is to educate yourself (check out Panda39's advice) on how to communicate and how to behave in a consistent manner towards him. All structure around him (and you ) has been turned over and stabilizing this situation will take time.
Did social service get involved against your will? Do you feel under time pressure?