V.C.
We have always had the teeth brushing battle with my son. (now 9) He has allergies and asthma and brushing his teeth gags him. We have talked with dentist many times and the dentist even give us some samples of different tooth paste. Nothing seems to work. One good thing about all this is his teeth are in better shape than his sister who does brush more but not as she should daily. Now that he is older almost 10 he is taking more pride in brushing. Maybe it is one of those things he has to do on his own time.
Have you tried rewards ? Example...Make a brushing chart for the fridge. (Sun.-Sat.) use a sticker for each day he brushes his teeth without a struggle. At the end of the week if he has all stickers he gets a reward. Doesnt have to be big things ... the dollar store has some really neat items or you could give extra priviledges ( never to early to learn this one) (movie,extra one on one time with mom or dad, trip to grandparents etc...)
Hope this helps .
V.