Hello S.,
I know what your going through, my daughter was the same way, and I know this may sound mean, but the best way to make sure that those little teeth get their cleanest is to hold him down. I once saw it on an episode of wife swap, and thought, oh my goodness, that's horrible, but then I had to try and brush my daughter's teeth, lol, and she's a fighter, so I laid her on the bathroom floor (on a rug) sat over top of her, not resting my weight on her though, and put her arms just below my knees so that my knees weren't on her, but she wasn't able to get her arms out and fight me, and it worked wonders, and now I'm able to brush her teeth without any fight, and while she stands in front of me. They may not like it, but until they learn that it's for the best, it's the easiest way to do it and make sure it's done right. I promise I'm not a mean parent by far, but I just don't want them to have rotten teeth. I hope this helps. Take care.
Candi ;O)