I just happened to read this, and I'd like to offer a bit of a different perspective. Carek, please understand that this is in no way a criticism of you or your son, just the way I tend to think of things like this.
And yes, I've experienced it myself to a small degree, and through other members of my family to a larger one. In fact, one of my nephews has been through almost this exact sort of thing with his ex.
I personally don't know what happened between the couple, or what your son was accused of, I can only go by what's in this thread. And I do understand that when you're in the midst of a situation, emotions can run high, and the desire for revenge can take over the best of us. But one thing to keep in mind through all of this is the welfare of the little guy, and the thing is, that his welfare is directly tied to that of his mother. In some ways, when she's being hurt, he will be too. Ultimately, your son did choose to have a child with her, and he (and maybe indirectly, you), will have to deal with this woman for the next 18 years. Wouldn't it be best to keep it as civil as possible? Maybe she's done things that are inexcusable, but IMO, sometimes it's worth being the bigger person in order to calm things down and smooth things over, and to establish some sort of a civil working relationship so that the two parents can be the best parents possible to the child. I firmly believe that a child needs both parents, if at all possible, unless there's something abusive going on, and that it's in the best interest of the child for both parents to be as accommodating to each other as possible.
I hope it all works out for everyone in the end, and especially for your grandson.