Hello K.,
When my daughter was 2, I assisted her / helped her with the brushing. I allowed my daughter to use the toothpaste that wasn't so hot for her. I think I allowed her to use that til she was, maybe 4? I don't really remember right now. But, assisting / helping your son has to be done on a daily basis, as you may know already. With the consistency of you showing him how to brush his teeth, he will eventually get it. It just takes time. He'll get it eventually.
So, I suggest that you brush your teeth during the same time he is brushing his. Maybe you can allow him to brush your teeth so that he will see why you want him to keep his teeth clean. It'll take time for him to catch on. You have to make it positive. You have to figure out what will work for you and him. I think, in order to get your son to brush his teeth, it should be repititious (spelling???) throughout the day until he gets it in his head. How about every time he goes to the bathroom. After he washes his hands, then he can brush his teeth. OH! You can make a chart with his name in BIG letters. Then on that chart, put Sunday, Monday, Tuesday, Wednesday, Thursday, Friday and Saturday. Next to each day, you can put up a small sticker of choice to reward him and praise him for his accomplishments. Then, maybe at the end of the week, you can buy him something fun, like bubbles and let him know that we're playing with bubbles because you've done so well at brushing your teeth. You know.....that's only a suggestion though. Be creative. Kids love to play. Make it a celebration. Let him know how proud you are of him, that he's cleaning his teeth so well.:)
I must say, that you still need to go over his teeth to make sure that there aren't any film on his teeth, because that will end up as a cavity. Also, on his permanent teeth, the dentist will put a sealer on them to keep them protected from cavities. My daughter has a sealant on her permanent teeth (back teeth). I wish I had that when I was a young kid. Even though my daughter is 11 today, I still look at her teeth just to make sure she brushed them thoroughly clean. We don't need cavities here. I want her to have her original teeth til she's 100. I hope she lives that long.
Take Care! And have fun with your son.:):):)