Been there....
This is always a difficult one. Its a long time since I sat on that side of the pointy end but I do remember that I had one particular Captain that I had difficulties with.
Can I offer this: You MUST take responsibilities for yourself, too. A little experience has taught me that the best time to breach the subject is after the day is done. Be direct and be succinct. Somewhere during the day, without him being around, make some notes and put them away some place.
Try and make it through the day and after the engines have been shut down and passengers are off the aircraft, turn to him and say: 'We've GOT to talk'..don't be afraid to 'lay it on the line'. Don't make any threats, but be diplomatic about things and try and get him to understand.
If that doesn't work and he goes ballistic...then, its time to get management involved. If that doesn't work....this will: Contact an inspector at your CAA and ask to talk to someone in the air safety department. Civil Aviation Authorities are under increased pressure to support air safety concerns, especially from ICAO and its oversite programs and the airline's IATA and its IOSA audit program.
I understand what you might be going through.