Using the model for My sons Benjamins circumstance

My son is 17 and does not see his dad. We had a vicoous separation and his dad was very abusive. He has chosen not to have contact with his dad and i have a lifetime restraining order. I have an excellent relationship with my mother in law and often have her over for dinner. She aometimes brings up bens dad in front of him ( my crazy and nasty ex) although i have asked her not to do this. Every time she does it, it generates a feeling of anger in my son because his thoughts are: she doesnt care about me, she has no idea how it hurts not to have a dad, she forgets about my feelings. This leads to the action of him leaving the dinner table and the result is that we all feel uncomfortable. I am trying to apply the model to get him to have a diffeent thought to generate more positive feelings towards his grandmother but i am stumped.