You poor thing, but if she going to go ballistic she will, regardless who he's with you or his sister.
This poor baby will be use no doubt, he needs to get to a solicitor for advise ASAP. If she has a similar history with a previous child this will be taken into account. Shared parenting is very common now , she wont get it all her own way. Any professional advise you go with him, they will give you both peace of mind and the way forward.
I have a young friend recently going down this sort of route sadly it got nasty before it got better. It's sort of resolved now but it has had a big effect on her parents especially her Mother, but now everything getting settled things are getting back to normal.
As his mum it will be hard to watch him go through this but he will be Okay, just continue the counselling session they will help you.
As for seeing you Granddaughter tomorrow , see how you feel on the day ?
Take care and I really hope they all sort it out soon.