The advise we would give at the dentist I worked at would be....
Give him a toothbrush to play with while you are with him, so it doesn't seem like some thing "foriegn" when you want to brush his teeth. If/when he puts it in his mouth say something along the lines of "are you brushing your teeth, what a clever boy"
Don't make a big deal of it, just let him play with the brush for a while. Let him see you brushing your teeth & ask him to help you, this way he will see it's not just something that happens to him. At first just use a wet toothbrush, then when you feel he as made some progress put a small spot of child's toothpaste on the brush. (We don't actually need toothpaste when we clean our teeth)
Don't battle with it too much as you could do more damage (to his perception of it, not his teeth) than good. Make sure you use a baby toothbrush, as the bristles will be softer & a baby toothpaste as it will taste more pleasent for him.
Until he will let you brush, just keep a close eye on his sugar intake.
As long as he doesn't think it just happens to him or that he HAS to do it, he will come around to the idea. Hopefully then you will have the problem I have, DD won't stop asking to brush her teeth! Everytime she uses the big toilet (and that's alot) she asks to brush her teeth!
Always have a seperate toothbrush to brush his teeth to the one he plays with. The play one will get worn out & won't do a good job when brushing. Then you can pretend that there are tweenies or fimbles stuck in his teeth (or his other favourite toys) that need to get out. Make it fun & it won't seem like a chore for him.
GOOD LUCK!