I am a clinical social worker turned community organizer with an 11 year old highly gifted son on independent study. We are working with our district to create gifted services so that we can get him back in school. He found middle school a waste of time (and now that we've tested him we can see why.) Our daughter has been bored in school for years, but she is compliant and social, so she makes do and excels anyway.
I would encourage you to read on gifted issues and on boy issues. The gifted field has tons of wisdom re: motivation and underachievement. (If you wonder if your very bright son might be gifted, read Losing Our Minds, by Deborah Ruf, Phd. GIfted children do not always choose school achievement. The boy also includes smart boys, who Ruf calls Level One gifted, since they have similar issues.)
Boys have different social-emotional-educational needs than girls and non-compliance with adult priorities is what adults feel is the "symptom" or problem, but its not that simple. Sometimes, we have to turn this subject of motivation inside out and ask first, what DOES my child enjoy and how is he doing overall? Also, is the school work meaningless to your child? If so, how can that be addressed? I was relieved when my husband agreed with me and my son that the problem was in fact, the school work. We are now working, as a team on a school solution, while working on our son's character issues too.
General recommendations for boys socio-emotional-developmental needs include: nurturing parents and caregivers, a clan or tribe, a spirtual life (may not be religion), important work, mentors and role models, to know the rules, to learn how to lead and how to follow, an adventure and a best friend to have it with, lots of games, an important role in life. (Micheal Gurion - and the gifted literature. Gurian is a controversial author.)
How is your son doing in these areas? Are there males in his life as mentors? What are his friends like? Are they motivated boys? In what ways? Has he found intellectual peers in his friends or is he sad because he doesn't have truly good buddies? I have had to turn over some areas of parenting to my husband recently, because our son needed male mentoring and this is working much better! Dad is being a role model, remembering also his own boyhood, and how he felt as an adolescent and sharing those stories, and the feelings that he had at those times, and how he grew to be responsible, all the same.
Those stories are important because they help the boy feel connected to another person, and understood and validated. The stories model identifying, expressing and coping with feelings, and showing character in spite of feelings of lack of motivation. Does your son have any goals for the future? If not, perhaps it is truly hard to see the value of school. I would also wonder whether school plays to his areas of strength. Motivation is usually highest in our areas of strength. Also, boys work harder when they feel connected to their teachers. The State Superintendent here stressed that "rigor, relationships, and relevence are needed in schools."
In our community, we have New Tech High, which is school work related to skills for the global economy and this setting sometimes helps young people find meaning, when they do not find it in academic subjects. Right brained youth (that's me) also find that school doesn't play to their strengths, so the parents job is make sure you understand your child's capacities and how school taps these, or not.
Again, the key is to realize (per the gifted literature, but applies to all youth) very few youth are actually unmotivated, they are just selectively motivated and/or not motivated in the ways that adults prefer. The solution is often to help them pursue what motivates them, with rigor, and then build a bridge from that, over time, to life skills, including school, and to show that even if they do not like school, success in school may be important towards their life goals.
Also, never forget that the chemical reality of adolescence - it is hard to feel good as an adolescent. Their dopamine levels are low. Their brains are not fully mylinated and smart kids make dumb choices all the time because their brains are literally short-circuiting. Their chemical state does not draw them towards school work, but towards interests that raise their dopamine levels. They start the day happy, are sad an hour later, and then happy again two hours later. It makes no sense to anyone - including to the boy or girl themselves. They need to know that they are on a developmental emotional rollercoaster ride of adolescence, and that we understand and will cut them some slack and give them support.
I hope some of this helps. It helped me to write it, reminding me to practice what I preach!