Hi D.!
I've taught high school for 15 years now, both private and public. Have you considered these points?
1) Is he "overinvolved" outside of school, with anything from athletics to community involvement? Does he have enough time to do his homework?
2) Is he unmotivated in everything, or just school? If it's everything, such as not enjoying his normal, favorite activities, you may be looking at depression or drugs.
3) Is he getting enough sleep and a good, healthy diet? Most teens need much more sleep and better nutrition than they're actually getting.
4) Have you chatted with his teachers? Perhaps they have some insight, such as struggling with reading or not knowing basic math. You'd be surprised how many students can fake their way through this until high school!
5) Can he see the board? Can he hear what's going on clearly? Perhaps hearing and vision tests are needed?
6) Is he involved at school? Students who are more involved (athletics, clubs, student government) tend to be more motivated.
7) Would he rather go to public school because he thinks it will be easier (not always true)?
I'm sure you've tried this, but, it may be worth trying again. Establish the rules: good grades equal priveleges. Bad grades = extra schooling (monitored by you), extra chores and no priveleges. In my family, those priveleges include tv, game boys, video games, computer time, friend time...all the fun things in life. This one is super effective with my two sons, ages 14 and 10.
Concurrent with privelege restrictions, you could try having your son research a possible career for the future. What kind of schooling does it require? Also, have him research how much it takes to live on your own (rent, car insurance, groceries, etc.) and have him figure out if he can live on minimum wage (or how much he needs to make to live a life with which he's happy). Those two lessons alone shock most of my juniors into motivation.
Last but not least, maybe it's time for your husband to step back and let you take over scholastic supervision for a bit. I had a student, brilliant kid, who purposefully graduated with a 1.0 GPA just to irritate his very strict father. Two years of junior college and he was in a top name university. <sigh> What a waste of time that was!
Anyway, I know this was super long and I'm sure you've tried all these things. Hang in there.
Regards,
S. F
PS: "Parenting with Love and Logic" may be a useful book, if you're interested.