I am also listening to what TheBigJessie is saying about how she feels about the pull of her mum (TBJ - does the fact that you know all this know and have acknowledged it make you feel better or worse? Was there a lightbulb moment for you?)
Not better, or worse. "Free" I think. It was kind of an odd drawn-out process. I knew by the time I was 15 I didn't see how she would be a positive influence on any grandchild, for example, and I also knew I wouldn't be able to stop her taking over any child if I was still living there. (I mean, you can't even claim child benefit in your own name for your off-spring until 18.) I was thus very careful and enthusiastic about contraception as a teenager! 
The difficult part was things like learning not to prioritise her over myself (and to entertain the idea that my mother's social analyses of modern Britain could be wrong- I used to repeat almost every thing she said as fact
). Recognising that her absurd conditions on things were her problem and I had no duty to sort it all for her. To draw an analogy, it was kind of like recognising that if ex-partner mother's says she'll only see a baby if it's a muslim baby, she's the one being manipulative.
Tbh, I regret posting on this thread in the first place, because I think I've just added to your terror. I only posted before, because I thought you were being overly-idealistic about the benefits of having a nasty, manipulative caaaaaaaaah of a granny in his life and being all soppy liberal.
I have never met your ex-partner; I'm not a psychologist. You need to talk to him about it all. And you should talk to a solicitor about child abduction concerns; the same way a married couple should have both their names on their house deeds, in case everything breaks down.
But it doesn't mean it will happen. Your ex seems quite intellectually aware of what kind of mother he's got and has known for a long time. He's even had therapy. He's been incredibly open about her, previously. Meanwhile, I have a sense judging from this thread- I may have completely the wrong end of the stick that you have been worrying about depriving your son of his grandmother all this time, and your ex knows that.
So, when the idea came up maybe after a nice weekend, when his mother had been on her best behaviour for the visiting guest, maybe he decided to consult you first, rather than dismissing it without you even having any input. You need to ask him, because he's an actual 4D person- width, height, breadth, and his own life-experiences, with his own strengths and weaknesses. Not a replica of me, not a replica of the latest report of a child-abducting father.
One more Yet another point: she's a nasty relation and most people have one, somewhere, I think but she's not in charge of the Pakistani Mafia, even if there was one! Even if she decided she wanted your child tomorrow, what's she gonna do? Walk in through your door and take him? Yeah, like you'd let her!
Her only access point (if she even wants it) is your ex. Who smokes, drinks, has relationships with whatever woman he falls in love with, no matter his mother's opinion...
See a solicitor. Get all the worse case scenarios sorted out, because that's responsible parenting. But don't take a little boy's father away from him, just because MN terrified you.