Perhaps you can work toward accomplishing two parenting goals in your efforts to address this challenge. Supporting responsible behavior AND creating a positive, fun-loving memory may be possible.
Natural and motivating rewards for responsible behavior (such as brushing teeth) may be the most effective. Discovering a reward that builds relationships is very positive for both you and your child. In other words, turn this potentially negative into a positive. Here are some suggestions that may be worth a try.
With a digital or camera phone, take a quick photo of each family member's shining" teeth (including mom and dad's) each morning they "remember" to brush. Make this a quick, playful, fun event. No picture is taken of "yucky" teeth and a "lecture" or "demand"to brush is avoided. If he forgets to brush, just say, "Oh, sorry!" "Don't forget to brush in the morning and we'll get your picture then." It will not ruin his teeth to go a single morning without brushing, and turning the requirement into an exclusive club that he wants to "join" may turn a battle into an adventure. Perhaps, only ones with super clean teeth get to see the pictures taken.
Before the novelty fades, change the reward up. Allow him to take your pictures, compete to see who can make the funniest "teeth" picture or have the biggest smile. Be creative.
Limit the photo to just one each person per day to keep it a special event/reward, save time, and keep the novelty interest high". Perhaps some days, his reward for responsible behavior will be a shoulder ride to the car as "King Responsible" or he can wear dad's/mom's sunglasses or carry dad's phone if he is very careful.
Night time brushing can become "practice times" for the morning photos.
The idea is to connect "responsible behavior" with special privileges and life/task related rewards and have fun, free of the "you've-gotta-do-this-because-I-said- so" demands.
This is also a great opportunity to have discussions about being responsible and how it means he is growing up and how proud you are of him.
As a parent, I usually thought of the negative consequence that would "pressure" my son into compliance because those were the strategies my parents used. I wish I had pursued ways of using the "carrot" instead of the "stick".
With fore thought and creativity, you can likely match other tasks to positive rewards and turn parenting struggles into family victories.