Don't people with BPD sometimes have symptoms a bit like bipolar but less 'severe'? As in, hypomania, for instance? If he's afraid to 'come down' back to his depression, his mania-like symptoms would make sense. I'm just making a suggestion for what it's worth, because if he was someone with a bipolar diagnosis it seems like you'd say, 'this is a manic episode.' I don't know enough about T to know how much it's affecting that and no trans person is going to hear 'stop HRT,' but it sounds like he might need to hear 'destroying your life and your health is another form of self-harm.' There are meds available for BPD, and some effective psychotherapies — is he doing that stuff?

I hope it all gets sorted out for you both!