K.J.
Would you consider hiring a coach for your son? There are coaches who work specifically with teens and help them understand the "what's in it for me" aspect of useful endeavors. They also help teens to create strategies that help them achieve their goals. Most importantly the coach will hold your teen to be accountable. Some teens have an easier time being accountabled to people other than their own parents.
Just a thought... and if you are interested, you can search for a coach at the International Coach Federation or in your local area.
If coaching is not an option, than consider how you can help him create his own motivation. He is more likely to succeed if he is doing something with his own buy in.
good luck