I'm going to take my grandson to lunch with my sons ex-wife without his knowledge or permission
Why are you going to do that?
She deserves to see him, and I'm tired of seeing her so sad when she can't see him.
It's good that you are going out places with your grandson, and that you care about all of the people in his life.  However, you shouldn't act without your other family members' knowledge or consent.  You might disagree with their choices, but you should respect them.
I'm just so sick of seeing her struggle mentally, and I don't care at this point about what anyone thinks.
Well, explain to your son that she is down in the dumps and needs some kind of pick me up. She needs to see her son too no matter what happened in the relationship. The grandson will resent the father if he doesn't let him see his mother too after awhile.