i had the WORST time getting him to let me brush his teeth, and felt like i had tried everything, until i stumbled upon this position, and it has worked since.
i put him in my lap, legs to the side, put my left arm around him like a hug, and lean him back on that arm, brushing with my right hand, somehow, the hug/snuggle-ness of it makes him less resistant. in the begining he was mildly resistant(an improvment) but he liked it that way and got used to it quickly, now jumps right up into the lap. Granted sometimes i have to really cajole, like no bedtime stories if he doesnt cooperate, and that usually(eventually) does it.
good luck, those little guys have a very big iron will!:)