J.P.
I don't know if this will be any help to you, but its worth a try.
To get my twin boys ASKING to brush their teeth I told them that we had to get the "monsters" out of their teeth! Suddenly they took quite an interest in it! Maybe try letting him pick out his own character toothbrush and paste. My 5 year old twins love the lightening bug brushes that light up for 1-2 minutes so you know how long to brush. (not to mention their cheaper than the character brushes). Maybe try a rewards/sticker chart. You could also sing a favorite song(ABC's, etc) while you brush and have that be your special toothbrush song, or find a special song about brushing your teeth.
I hope this helps! Good Luck!
J.