Honestly, it seems like there is something else going on here. What are his othe symptoms? Paralalia, at least I thought, is common in schizophrenia. Also, if he is having hallucinations, (although possible in bpd) it seems like these together might be suggesting something else. Of course, I'm not saying he has schizophrenia, just perhaps another aspect to his illness that hasn't been considered. You should discuss this with his doctor.
From what I remember, auditory hallucinations seem to come from Broca's area (which you could google), at least in patients with schizophrenia. It involved in speech and language processing. But the brain is so complicated it's always hard to tell exactly where something comes from, it's all connected.