I am in the same position as you but we already have marks on iains top front 4 teeth from the lack of fluoride ( we are on tank water)
I do the same as you atm. Pin him down & just get it done. Its been the same with all the kids but they get there. What brand of tooth paste do you use? My kids say Colgate is too hot, even the kids one. I get the macleans one with the wiggles on it. They don't mind that one. I am also told that you get ones with no flavour at all but I am yet to find one. The other thing I do is ditch the brush & just use a wet face washer with a dab of tooth paste on it over my finger. But I usually have to swap back & forth between techniques.