I can understand why you're so upset, unbeliever. I can also see why your baby's father doesn't "get" how serious this is too.
Maybe ask him how he feels about being used and manipulated by her to get what she wants, in complete defiance of your expressed wishes. That's not a position anyone would want to be in. The fact that he trusted her to go on an outing with the baby actually says something to me about him. Not that I know how well he and your mother know one another, but still, I would question his judgement in allowing someone you are cautious about to have unfettered access to your child.
If you had said to baby's dad: "I do not want the baby to be exposed to tuberculosis, because he is a baby and his immune system cannot handle the exposure" would he have gone against your wishes? I doubt it. When you boil it down to the basics, there's not much difference between exposing a child to a contagious life-threatening disease and exposing them to insidious, alienating, mind-altering doctrine. Both have the potential to destroy your child's life and your relationship with him, because a child does not have the capacity to distinguish between reality and the fiction of religious doctrine. Your wishes, as the mother, deserve to be regarded with the utmost respect.
That being said, you really need to make it clear to your mother and your baby's dad that she is not to have access to your child unless YOU are there to supervise the interaction. She's made it plain that she is not trustworthy and that she needs to be supervised.
I would make her wait a few months before she gets a chance to see your son again and make sure she understands that you are in charge of your son's values and belief system, not her.
If my non JW grandparents ever tried to take us to church (or even to an event in a church), my JW parents would have gone through the roof and be just as outraged as you are now.
The bottom line is that it is not up to the grandparents to usurp the parent's role and decisions. That is a boundary violation and it has to stop.