I agree with Annie all this yelling and threats are harming ,so back off , allow him some leeway of when his teeth must be brushed his bathing,etc its his body let him have more control .You are the adult, you are arguing, If a lot of this happens in the morning getting to school allow more time, children are half asleep they hate being rushed around many parents are frustrated by that morning race, set the clock earlier ...
It's not surprising he's learned to call the kids at school names if you yell at him every night and threaten to send him to live with his grandparents. You're his role model, after all. If "he also recently told me that I ruin his life, which I, as his mother, only want whats best for him," well, yelling at him every night and threatening his existence at home by telling him you'd like to send him away *IS* pretty much by definition ruining his life. At age 8, with puberty looking them in the face, what a kid needs to know the most of all things is that no matter how much he changes or goes through emotionally, he will never be rejected by his parents.
Please start (like tomorrow) with the book "How to Talk so Kids Will Listen and Listen so Kids Will Talk," and go on from there to fix your own patterns that lead to yelling. You should be starting your son earlier to get ready for bed, and give him some options and rewards, at 8. He's certainly old enough. When my nephews were that age, for example, I would tell them they could sit up reading as long as they wanted, as long as they had brushed their teeth and were in pj's and in bed. They liked the tradeoff, and so did I, since they always conked out pretty fast after they were there. :)
Please also see a children's therapist for ideas for yourself on how to get past this bad pattern. He's just being 8, and boundaries set by parents are an issue from now until his late teens, you may as well learn some better ways to communicate now.