What does your mom do different? Do you think he just knows you will give in and give him a bottle and he likes the mommy time he gets?
I would harass my doc if I was you. Are you sure he is not gaining? They dont gain a whole lot at a time anymore? I wish I could help. I would try toast with different toppings, yogurt, hummos jam etc to see if he like it. Keep trying...it will happen.