P.F.
Ahhh, I just sent you a long email on how we handled this with my step son and it didn't go through. Here's the short version of what it was...
Your son is going through what I've watched my step son go through at the same age. On trying to figure out what to do with him...every male in my life told me they went through the same thing. There is nothing wrong with him, he is going through adolescence. It's not cool to be "good" all the time anymore.
My father in law did this to my husband and it worked, we did the same to my step son and it worked. He was getting straight A's and he was always in accelerated classes but in Jr High he went to being a pain in the butt and struggling with horrible grades. Here's my version of what my husband and I did after wrestling with him for a year and a half.
We sat down together and told my stepson...These are your grades not ours, we've already made our grades and are successful in life. From now on, your grades are your decision and your responsibility. The world will always need people to flip burgers for $5/hr.
The next step is to forget everything that happened up till now and start fresh. Don't get angry if he does bad, just ignore it...don't make any snide comments or it will all unravel.
Next,learn how to discreetly get him to talk about school. How was your day? Ok. Oh yeah? Did you have any fun assignments? blah blah. Just start with something he likes and slowly connect it with school so you just naturally end up talking about school.
Then, praise him no matter what! Was the C better than last weeks D? "That's great, you've brought your grade up!" Was the C worse than last weeks B? "Aww, that must have been really hard, I used to have trouble with that too."
Have him get involved in school clubs or activities that HE enjoys. These will cause him to get connections that make him want to be in school. Also, most of them require you get good grades in order to stay in them.
Reward him when he does good without announcing your good deeds. He's doing better...spend alone time with him, Dad build a rocket with him, take him for an ice cream cone when you pick him up from school, let him stay out a little later this weekend, talk about his hobbies, buy him a small gift.
This turned my step son around immediately, he was back on top and wanted to get back into accelerated classes as well as start planning for the future (an added bonus!). I'm sure it will help you as well!