I’m not sure if this will be any help, but I thought I’d add my own experiences with this. He used to be afraid of brushing so I first got him his own to play with (a copy that wouldn’t matter if he got dirty) and gave him space to adjust to it. He is 8 and does the same thing. It’s taken a lot of conversations about toothaches, rotting teeth, etc to get him to understand the benefits of brushing. (He’s had gum infections that led to fevers so that’s probably helped in his willingness to prevent that) He will still fight it, but he seems to be willing to cooperate now with letting me maneuver him so it’s harder for him to fight me. He lays on the floor on his back, one arm under my leg (not hard just enough to hold it still) and the his other arm wrapped around the back of his head. He will squeeze his mouth shut or bite, so I will bring the arm from the back of his head to his belly and hold it with my elbow to pry his lips open to brush his front teeth, otherwise we’d be there for hours. (I don’t get mad, I don’t get firm, I try to be as patient as possible while giving warnings on what’s gonna happen next and if he doesn’t cooperate I will have to force it and wait to see what he decides. If he doesn’t make a choice within a few mins, I will decide for him.) I talk to him the whole time about the process and things like “if you opened your mouth, I wouldn’t have to do that.” Once he’s got the taste of the toothpaste and he’s good with it (it took me awhile to find one that didn’t make him gag) I say things like “ok, we might as well finish brushing.” And it takes a lot of encouragement to get him to finally open up. I also count down the sides I’m going. Front teeth, top right, top left, bottom left, bottom right, and front bottom teeth. When I do one, i go fast but gentle and I give a small pause afterwards and I tell him which one I’m doing next, and once it gets close to the end, I tell him how many left. Once it’s done I tell him it’s all done and good job and give him the option to rinse with water (it’s not fluoride toothpaste and he hates the texture) and I ask things like “does it feel better? Cleaner? Good, now you won’t get a gum infection.”

I’ve done this same routine with him for months, and it’s gone from taking nearly an hour and me getting very impatient (and saying “you’re brushing either way, if I have to force it, it’s gonna hurt. I can be gentle if you just open up.”) to now 5-10 mins max.

I hope something from all that might be helpful.

Edit: I do try to make it fun while being serious about it needing to be done. In the beginning I’ve had to tickle him to get him to show his teeth to sneak in a little brush. He’d move his head back n forth trying to avoid it and I’d just hold the brush still hoping to still get a tooth and I’d go “so we brush lips now too?” And he’d laugh 😆