Power strugglwa over important issues are hard because sometimes the more they resist our help, the more we try to show them how important something is.,..tug tug..
Try this: there are great little books on toothbrushing, just reread one last night..shows the steps..even the plague guys (we call them sugar bugs) ...there is a poem on how much toothepast, how it is yummy but not for our tummy, how to go up and down, back and forth.and you read it not at brush time and not during the struggle...then that time because his own as he figures it out.
Brush in front of him..the whole routine
Find cartoons (dora, diego, noggin, blues clues) on the topic..they love listening to the characters..and their peers..if a cousin or neighbor is over..let them teach them..
Praise him for being a great big boy brusher and do it more through out the day and ask him for when he needs help....
Lastly, my son is visual so I treat the mouth like a house, the front teeth or the top windwos and doors, the back teeth are bedrooms, the bottom are the kitchen, the living room, etc...he let me go in and make it fun...same with my daughter...
happy brushing