M.S.
Tooth brushing should be a daily habit by this age, and should be non negotiable. I sit my son on the counter and brush his teeth no matter how long it takes and how many times I have to tell him to please open his mouth. When it first became difficult, I would brush my teeth in front of him and then he wanted his done too (with my electric toothbrush). So once we established that as a key to getting him to cooperate, I bought him his own kids electric toothbrush which he loves. I do not let him brush his own teeth or chew on the toothbrush. Once I'm done, he can have a try at brushing, but once it becomes chewing, sucking, spraying spit/toothpaste everywhere, or leaving the bathroom, I take away the toothbrush and we're all done. It might be a battle for a while, but it's one you need to win. :)