I think it would bother me if I had to hide something like that from my child too.
You have to do what is best for your grandson.
If it is not safe for you to tell him that baby momma is coming to visit than you got to do it to protect your grandson.
I am not sure but your son may feel like you are on her side more if you don't tell him and he finds out on his own.
Sounds tough for sure. Good luck