Mleigh4, as I read the posts above, the word that came to my mind was "control."
I could be way off base, but when you expressed your displeasure about Grammy cutting S's hair, you reminded her that you control what happens or doesn't happen as far as raising your S and it's not her place to interfere without running things by you first. Obviously, your h plays a part as well, but S lives with you and you have become the primary parent in the everyday stuff that goes on in your S's life.
It looked as though you and h were doing okay in the co-parenting department until MIL got in h's ear.
From the outside looking in, it looks as though MIL is using your h to try to gain control that does not rightfully belong to her. She obviously knows what buttons to push to get your h all riled up.
Does MIL know your S doesn't want to go over there? That could add to it, too.
In the long run, you have to do what's in your son's best interest and this kind of animosity between the adults in his life is not good. Job is right. You have to nip it in the bud.
I know ... easier said than done.
{{{Mleigh4}}}
Me: 59 and holding H: :53 Me: 1 S, 1 D, both grown M: 19 T: 23 BD: 9-23-2013