If you think it's BDP, ask him to see a doctor and get a diagnosis. The only way you will know is if you get a doctor to diagnose him. Otherwise, it's a waste of time to try to speculate and think of therapeutic solutions and diagnoses on your own without true tests or a doctor's input. I don't agree with the advice to start a DBT workbook without a diagnosis from a doctor. He could also be a toxic or abusive personality.