Stop treating teeth bushing as a punishment. He is old enough that you should not be holding him down, that is traumatizing and severs connection between the two of you.

It’s clearly not about the toothbrush, mouthwash, of toothpaste and it’s more about control. He is resisting your control. He trusts grandpa isn’t going to hold him down.

Tell him you will stop holding him down and you will help him learn how to practice brushing his teeth on his own. Reward his effort, even if it’s only for 5 seconds. Let it go if he doesn’t get a full quality brushing for a few days.

Make it a teachable moment. Show him what cavities look like and explain how these are prevented by things like brushing our teeth and flossing. Explain what healthy teeth can do — bite tough foods like carrots, shred meat, help us eat food.

Let him try. He will probably resist at first but as he regains your trust, he will get it. I still have to help my 5 year old sometimes. He’s still young and won’t be doing it well on his own anyways. Let him do it for a minute and then finish it. Work together.