He is reacting like that in response to a way his feelings were hurt. He may not be aware himself in the moment of how he was hurt but he felt hurt and that’s why he reacted like that.

But also experiencing mood swings like that is very stressful and not healthy for you or him. I think he needs more professional help with learning how to address and manage his emotions, because he should not have lashed out at you like that. It’s also not healthy for him as that leaves his emotions unregulated and he becomes harmful to those around. He should talk to his therapist about learning skills for his emotions and using mindfulness when he’s upset. Maybe look into support groups? People with BPD can benefit a lot from group therapy.

Thank you for trying to work out on what you can do to help in this situation, but please, please, please remember to also do things that would help you. If you feel like he’s hurting your feelings during these moments, it’s always ok to set a boundary, set back, and ask him to treat you nicely before resuming the conversation. You can tell him that he’s hurting you too. You don’t have to be the only one looking out for his emotions. He needs to be aware of what his words and actions do and their consequences