Not trying to be a dick here, but maybe he's so in "love" with this girl because of his home situation. If his dad really is the douche you say he is, then your kids can see it, and may be looking for family in other places.
Maybe it is time to let blueshirt into his life some more. A teenage boy would probably listen to a grown mans advice.
Another point of view from an adult male could be very valuable IF the kid respects him. Ha ing a shit pile for a dad is definatley an influence even if he isn't concious of it. I agree with Woody, get the bf involved.