Normally, when a man and a woman who have been in a long term committed relationship and were already on the road to marriage find themselves expecting a baby, I am the first one to say, "Get to the JP already and make it official before that baby is born." But in this case, I cannot say that.
If you were my relative, I would say to you, "Do not marry him until he grows a backbone and is able to stand up to his mother, defend you and tell her in no uncertain terms that if she continues to treat you in this manner, she will no longer be a part of his life. Because if you do marry him, knowing he cannot and will not stand up to his mother and cannot and will not defend you when she goes after you, then you are consciously agreeing to years of conflict and misery. Do you want that for you and your baby?"
There are many overbearing, manipulative mothers out there who have raised sons who lack the ability to stand up to them.
God help the women who marry those men. On the other hand, there are men who have a lightbulb go off in their head and say to themselves, "I've put up with this crap from mama for years, but if I don't make her stop, she may cost me my girlfriend/fiance/wife." And they decide that they don't
want to lose that woman, so for perhaps the first time in their lives, they tell mama she needs to back off and respect the woman in question or there will be consequences. If he sticks to his guns, his girlfriend/fiance/wife will have a lot of respect for him and their life together will be a lot happier.
I know what I am talking about. My MIL was a master manipulator. She was the only woman in the family and played the men like fiddles. My DH, among others, hated confrontation and conflict and so he (and the others) would give in to her games and fits just to shut her up and keep the peace. It seemed easier. Then DH and I got married. She had issues with me from day one because I come from a long line of feisty women who take no crap.
And I wasn't going to take hers. So she slowed down with the manipulation. But after a few years, a family crisis hit and she used it to issue an ultimatum and essentially make DH choose between her and me. (Not for good, but to do what she wanted would have been a clear "he picked his mother over his wife" and would have given her a power position.)
Because of the crisis, it was downright cruel to put DH in that position and make him choose. I told him I wasn't going to add to his stress and guilt trip him the way his mother was and that he needed to make his own choice. He said, "The choice was made a long time ago." I asked if he meant when we got married and he said, "No, it was made the day I
asked you to marry me. She's just going to have to deal with it." He called her and let her know he would not be giving in to her demand.
She had not expected that. We saw her a few days later and she dang near grovelled at my feet. She was sweeter than sweet. She had tried to make him choose between her and me.......And he had. It just didn't go her way.
For the next 20 years, she never pulled a stunt like that again. (At least not with us...She kept manipulating everyone else.) My DH, a man who hates conflict, had stood his ground and made it clear that I would always come first and there was no choice to be made. If she tried, she'd lose. I'm almost grateful she made that power play.
But I've had friends whose DHs have not managed to stand up and make it clear their wives were #1 and it has been a
never ending source of arguing, anger, resentment, etc. I agree with the posters who said this is
NOT between
YOU and his mother. It is a problem between
HIM and his mother, in that he needs to step between you two (literally and figuratively) and say, "Back off, Mother. I won't tolerate this from you." If she continues, make it plain he will cut ties. And mean it.
It boils down to this. He can only have
ONE "most important woman" in his life. Who is it going to be?
You or his mother? 
Until the answer is
YOU, don't marry him.
