3 tips

Video distraction, the music in the video was about brushing teeth

The only position that really worked to brush his teeth was he lying on the bed, using my legs to hold his furious arms. After a while he learned it didn't hurt and accepted it.

We got a dinosaur and ask him to brush its teeth. When he tried, we close the dinos mouth to show how difficult it was. We then asked him to ask dino to open his mouth. So he did. We opened dinos mouth and he brushed dinos teeth.

After a while somehow he learned he needed to cooperate.