Okay here is what you do - you tell him to get his mother to stop. easy as.
As a man i would never allow my mother to do that to my wife, ever. and since she is my mother it would be my responsibility to control the situation, if things got that ad i would ban her from visiting my house.
If he isnt willing to stand up for you , then he is a weak man.
then you need to make an appointment with his mother in private - and simply say to her that if this behaviour continues , it will cause a family fall out. and ask her who she thinks her son will choose. if she says her - tell her you will ask her again when you are pregnant. (not that thats the plan)
its sad - this is a man who is suposed to be willing to die for you, but he is still scared of mommy. its a crying shame.