With regard to the idea of dogs loving their abusive owners (and by extrapolation, children loving abusive parents), it's got to be more complicated than that - and of course the other adults in any child's life have a duty to try and prevent any abuse from occuring.
If you think about it, a small person is rarely going to have the confidence or vocabulary to articulate the confusion of feelings that they must experience when someone they love abuses that relationship. I can't speak from experience here (thank god) but would imagine many adult survivors of childhood abuse or people who witnessed domestic violence in childhood would agree that their feelings towards the abusive/violent parent were always ambivalent - even if they couldn't put their finger on it at the time.
In the end, we put the label of "love" on the feelings that children have towards their parents, but there must be a real mix of loyality, dependence, habit, fear, confusion, guilt, etc. in the case of children who live in these circumstances.
So, in short, I think GF that you are right to protect your DS from being pulled about. I wouldn't beat myself up about it if I were you. But it might be good to try and encourage open dialogue with him when he is at the right age, to help him come to terms with what is, in effect, the loss of a parent.