I would be inclined to leave him to it.
It's a difficult situation - you want to help your dad with his issues, but confrontation, and it will be confrontation, may not be the best way, and you may not be the best person to address the issue.
As a parent, I would feel less inclined to accept intervention from my children than just about anyone, however well intentioned I know it would be.
Having someone whom you have to taught a moral code from the day when they could begin to understand what a moral code is, then turn around and address your own moral shortcomings is a uniquely difficult situation to deal with, which is why I would suggest that you are not best placed to help out here.
If there are any other close family members apart from his children who could talk to him, I would suggest that is a better option for you and your dad, to avoid damaging your relationship with no end benefit in sight.