The answer has to be by showing him choices and rewards.
Ask him what he wants, he'll want a car, get the car section out of the paper and get him to cut out the cars he wants and put them in an envelope marked "cars". You will then cut out the old bangers and put them in your envelope called "cars".
Do the same for all sorts. Houses, holidays, hobbies.
Then get the jobs section out and start talking about what job he would like or could do. Slowly, go through the jobs he can't do without an education and start to open the envelopes demonstrating what he can have in the lower paid jobs and what he can have with the higher paid jobs.
Then look at what's needed to achieve the higher paid jobs.
Then leave him to it to think about it. Give him all the envelopes and let him walk away to think about it.
I never understood that doing well at school impacted on your whole life. That it limited all choices.
He should then carry an envelope of the things he wants in his pocket and tell him to just touch the envelope or flick through the pictures when he felt annoyed and unfocussed.