I wasn't suggesting to "start the DBT workbook". My partner's psychologist/therapist recommended this workbook, judging by the concerns he expressed in their sessions. DBT was designed for BPD but is not limited to. You don't have to have a formal BPD diagnosis to use this workbook.

If you ARE interested in it, I'd talk with him and see if he'd be interested in booking an appointment with whomever (doctor, psychologist, therapist) and see if it's a good fit for him. At the very least, it'll open up a conversation, one way or another, and you can express what behaviours you're noticing.