I've posted about this before and maybe you can find it in the archives, but here is what I did for my daughter (who would fight and BITE ME when I brushed her teeth!)
Get some stickers or other small toys that are age-appropriate (I like stickers cause they are CHEAP!)
After brushing his teeth, leave a sticker (without him seeing) on his pillow, or even in his pocket, etc. Then say, "Look... the Sticker Fairy must have seen you brush your teeth!"
You can make it as elaborate as you want (sprinkle glitter or confetti, etc. during the day to say, "Oh the Sticker Fairy must be waiting to see if you brush today!!"
I left a bigger gift after a dental checkup too.
Another thing that may work is to get him a battery powered toothbrush and let him pick it out. They clean better AND they make all kinds that look fun. You can get a small sand-timer to let him turn it over while he brushes and you brush OR use a kitchen timer, etc.
Good Luck! I know it is a PAIN to hold them down and brush!
-Amanda