Unfortunately, I think it wouldn't accomplish anything and you'd just be wasting your time and draining yourself emotionally and physically. Chances are they already know he has a drinking problem and that he's manipulative and they'll just end up resenting you.
Perhaps, deep down, you have a different motive. Perhaps you're not ready to let go of your partner yet and staying in touch with his family is a way to keep your foot in the door. Something to ponder.