I have been called poo poo head, silly old woman, I hate you Granny! I have been kicked, nipped and pinched, slapped. I absolutely love my DGS to bits, he has Asperger's syndrome. The behaviour of your grandson sounds very like mine, he has often told me that he likes his Grandma better than me.
He is almost seven now, and has started to gain some control over his worst behaviour. He still has "meltdowns" but now retreats to his room, or a quiet area at school until he feels in control.
I hope it is not a lack of discipline, and poor parenting that causes this behaviour in your GS. How is he at school, and with his friends? Does he have siblings? How is he with them?
I'm sorry that he is this way with you. I hope what I have said above helps. It's something to think about anyway.