You are in a relationship with a man that you need to fix to make the relationship function properly.
If his mum is abusive then he will be affected by this, at best he will never be able to put you first (fear, obligation & guilt) at worse he will become like her. This is what you will struggle to believe as I am sure he is nice now, even if he doesnt care about your feelings sometimes.
You are young and if you tie yourself to a "problem" man that you have to fix to make you happy then you WILL miss out on the emotionally healthy men out there. When your life is full of pain, drama and conflict you will look at your friend's lives and envy those who have emotionally stable husbands.
This is what I did, took on a man who had an highly abusive mum. Of course I thought love would conquer all and he would get better but a massive warning is that HE isn't at the stage of knowing it is a problem. Without insight and motivation it will not get better.
If she is truly disordered then the impact on his emotional development will have been severely hampered. You often can't restore this development as brain structures have been affected and normal counselling doesnt work. Ex has years of counselling and nothing worked and it gets worse as they age. My physiologist said "troubled childhood, troubled midlife".
Ex had lots of good points, reliable, hard working and seemed to adore me but he inherited his mothers traits. Nurture or nature I am not sure however I never imagined he would have got so abusive as he did.
You may ignore the warnings as youth makes us optimistic and hopeful but in matter of love choose very, very wisely. A man's relationship with his mum is often the blueprint for female relationships. Ex hated women although it was not something he was very conscious of, he also had deep anger that he had to repress and that only came out when he had achieved some power through a career.
His mum was awful and I did feel sorry for him..it was the reason I stayed but love does not fix a bad childhood. Your bf has to work it out for himself as that gives him the best chance of recovering.