I don't think your problem is with his mother - it is with your so call fiance.
He is not standing up to her or up for you.
You should have ran when he gave you the pre-nup - not have a baby with him.
This means you saw him - who he truly was, his essence - before having the child and you thought, maybe you could change him?
Also, you bent so much - so far - I don't see how you are going to stand up to him now.
I am for marriage and I always say work it out, work it out, but in this situation your man sounds selfish, small, cheap and he's counting on his mother living longer than both of you to take care of your son? He sounds dumb too.
Remember his mother has a man and is living her life. He is the one refusing to live his life. Your anger is misdirected. I would be so angry at him not trusting the love you guys share and not setting boundries in your relationship. That is his job.
The devil can knock on the door, but you do not have to answer.