What should I do if my father chooses his new and very abusive wife over me and his grandson? Should just cut them out of my life or stay and watch and have to deal with his wife mistreating all of us

The first thing you need to do is to talk it out ,confront your father, speak your heart out, if that does not work, I think you need to listen to your heart and see what you want to do. Consider what is the best thing for the well being of your child and yourself. Maybe even distancing yourself from them for a while, because, its true when they say absence makes the heart grow fonder. Don't let yourself bear the injustice that is being forced on you, stand up for yourself and your son. It brings allot of heartache ,i understand.But trust me , it will work out.