I'm thinking of cutting my mother-in-law out of my sons life
Why do you want to cut her out of your son's life?
She doesn't bring anything positive to my son's life and she's pretty toxic.
That's her grandson she should be allowed to see him. You should just talk to her about what she is doing that's harmful.
It's getting harder for me to stand what's happening.
I hope that you can find a compromise here that protects your son but keeps his grandmother in his life. Can your spouse be the go between, to try to set up boundaries to help with the toxic behavior?