Give him an ultimatum - either he:
a) starts taking care of the dog
b) relenquishes all responsibility of the dog to you and your bf
If he refuses either of these options then I would threaten to call the humane society.
Tell him you really have no choice, if he isn't going to claim responsibility then you can't stand to see the dog suffer. You don't actually have to call the humane society, but see what happens when you threaten it. Worse comes to worse the dog can always "run away" to your house
