I take it that he just is lazy, and doesn't want to take the time to do it. Tell him as a teen boy, believe it or not the teen girls pay attention to it. I have a 15 yr old daughter, her and her friends actuall y make fun of people whose teeth aren't brushed. I have heard it first hand, and it doesn't only happen once. It happens everytime they see the plague on their teeth and gums.
I would also find on the internet pictures of people whom have terrible gums, and teeth due to not brushing them. Maybe he will take what I said about all the teen girls that come to my home, and tell me of the guys whose teeth are gross, and then the pictures you can find on the internet will wake him up>
Another suggestion, I would not allow any sodas, candy, sugar type cereals, fast food, ice cream etc, and tell him until he brushes his teeth each and every day those foods will not be allowed PERIOD !!